lr.py

# Copyright (c) 2020 PaddlePaddle Authors. All Rights Reserved
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

import math
import numpy
import warnings
from paddle import Tensor
import paddle.fluid.core as core
from ..fluid.framework import _in_legacy_dygraph

__all__ = [  # noqa
    'LRScheduler',
    'NoamDecay',
    'PiecewiseDecay',
    'NaturalExpDecay',
    'InverseTimeDecay',
    'PolynomialDecay',
    'LinearWarmup',
    'ExponentialDecay',
    'MultiStepDecay',
    'StepDecay',
    'LambdaDecay',
    'ReduceOnPlateau',
    'CosineAnnealingDecay',
    'MultiplicativeDecay',
    'OneCycleLR'
]


class LRScheduler(object):
    """

    LRScheduler Base class. Define the common interface of a learning rate scheduler.

    User can import it by ``from paddle.optimizer.lr import LRScheduler`` ,

    then overload it for your subclass and have a custom implementation of ``get_lr()`` .

    Otherwise, an ``NotImplementedError`` exception will be thrown.

    Args:
        learning_rate (float): The initial learning rate. It is a python float number.
        last_epoch (int, optional):  The index of last epoch. Can be set to restart training. Default: -1, means initial learning rate.
        verbose (bool, optional): If ``True``, prints a message to stdout for each update. Default: ``False`` .

    Returns:
        instance to schedule learning rate.

    Examples:
        Here is an example of a simple ``StepDecay`` implementation. 

        .. code-block:: python

            import paddle
            from paddle.optimizer.lr import LRScheduler

            class StepDecay(LRScheduler):
                def __init__(self,
                            learning_rate,
                            step_size,
                            gamma=0.1,
                            last_epoch=-1,
                            verbose=False):
                    if not isinstance(step_size, int):
                        raise TypeError(
                            "The type of 'step_size' must be 'int', but received %s." %
                            type(step_size))
                    if gamma >= 1.0:
                        raise ValueError('gamma should be < 1.0.')

                    self.step_size = step_size
                    self.gamma = gamma
                    super(StepDecay, self).__init__(learning_rate, last_epoch, verbose)

                def get_lr(self):
                    i = self.last_epoch // self.step_size
                    return self.base_lr * (self.gamma**i)

    """

    def __init__(self, learning_rate=0.1, last_epoch=-1, verbose=False):
        if not isinstance(learning_rate, (float, int)):
            raise TypeError(
                "The type of learning rate must be float, but received {}".
                format(type(learning_rate)))
        self.base_lr = float(learning_rate)
        self.last_lr = float(learning_rate)
        self.last_epoch = last_epoch
        self.verbose = verbose
        self._var_name = None

        self.step()

    def __call__(self):
        """
        Return lastest computed learning rate on current epoch.
        """
        return self.last_lr

    def step(self, epoch=None):
        """

        ``step`` should be called after ``optimizer.step`` . It will update the learning rate in optimizer according to current ``epoch`` .
        The new learning rate will take effect on next ``optimizer.step`` .

        Args:
            epoch (int, None): specify current epoch. Default: None. Auto-increment from last_epoch=-1.

        Returns:
            None

        """
        if epoch is None:
            self.last_epoch += 1
            self.last_lr = self.get_lr()
        else:
            self.last_epoch = epoch
            if hasattr(self, "_get_closed_form_lr"):
                self.last_lr = self._get_closed_form_lr()
            else:
                self.last_lr = self.get_lr()

        if self.verbose:
            print('Epoch {}: {} set learning rate to {}.'.format(
                self.last_epoch, self.__class__.__name__, self.last_lr))

    def state_dict(self):
        """

        Returns the state of the scheduler as a :class:`dict`.

        It is a subset of ``self.__dict__`` .
        """
        self.state_keys()
        state_dict = {}
        for key in self.keys:
            if key not in self.__dict__:
                continue
            value = self.__dict__[key]
            if isinstance(value, Tensor):
                assert value.shape == [
                    1
                ], "shape of Tensor in state_dict must be [1] {}".format(
                    value.shape)
                value = value.numpy()[0]
            state_dict[key] = value

        return state_dict

    # For those subclass who overload LRScheduler, "last_epoch, last_lr" will be saved by default.
    # (Note): you can change it for your subclass.
    def state_keys(self):
        """

        For those subclass who overload ``LRScheduler`` (Base Class). Acquiescently, "last_epoch, last_lr" will be saved by ``self.keys = ['last_epoch', 'last_lr']`` .

        ``last_epoch`` is the current epoch num, and ``last_lr`` is the current learning rate.

        If you want to change the default behavior, you should have a custom implementation of ``_state_keys()`` to redefine ``self.keys`` .

        """
        self.keys = ['last_epoch', 'last_lr']

    def set_state_dict(self, state_dict):
        """

        Loads the schedulers state.
        """
        self.state_keys()
        for key in self.keys:
            if key in state_dict:
                self.__dict__[key] = state_dict[key]
            else:
                raise RuntimeError(
                    "Please check whether state_dict is correct for optimizer. Can't find [ {} ] in state_dict".
                    format(key))
        if len(state_dict) > len(self.keys):
            warnings.warn(
                "There are some unused values in state_dict. Maybe the optimizer have different 'LearningRateDecay' when invoking state_dict and set_dict"
            )

    # alias for set_state_dict
    set_dict = set_state_dict

    def get_lr(self):
        """

        For those subclass who overload ``LRScheduler`` (Base Class), User should have a custom implementation of ``get_lr()`` .

        Otherwise, an ``NotImplementedError`` exception will be thrown.
        """
        # calculate by python float
        raise NotImplementedError


class NoamDecay(LRScheduler):
    r"""

    Applies Noam Decay to the initial learning rate.

    The algorithm can be described as following.

    .. math::

        new\_learning\_rate = learning\_rate * d_{model}^{-0.5} * min(epoch^{-0.5}, epoch * warmup\_steps^{-1.5})

    Please reference `attention is all you need <https://arxiv.org/pdf/1706.03762.pdf>`_


    Args:
        d$_{model}$(int): The dimensionality of input and output feature vector of model. It is a python int number.
        warmup_steps(int): The number of warmup steps. A super parameter. It is a python int number
        learning_rate (float): The initial learning rate. It is a python float number. Default: 1.0.
        last_epoch (int, optional):  The index of last epoch. Can be set to restart training. Default: -1, means initial learning rate.
        verbose (bool, optional): If ``True``, prints a message to stdout for each update. Default: ``False`` .

    Returns:
        ``NoamDecay`` instance to schedule learning rate.

    Examples:
        .. code-block:: python

            import paddle
            import numpy as np

            # train on default dynamic graph mode
            linear = paddle.nn.Linear(10, 10)
            scheduler = paddle.optimizer.lr.NoamDecay(d_model=0.01, warmup_steps=100, verbose=True)
            sgd = paddle.optimizer.SGD(learning_rate=scheduler, parameters=linear.parameters())
            for epoch in range(20):
                for batch_id in range(5):
                    x = paddle.uniform([10, 10])
                    out = linear(x)
                    loss = paddle.mean(out)
                    loss.backward()
                    sgd.step()
                    sgd.clear_gradients()
                    scheduler.step()    # If you update learning rate each step
              # scheduler.step()        # If you update learning rate each epoch

            # train on static graph mode
            paddle.enable_static()
            main_prog = paddle.static.Program()
            start_prog = paddle.static.Program()
            with paddle.static.program_guard(main_prog, start_prog):
                x = paddle.static.data(name='x', shape=[None, 4, 5])
                y = paddle.static.data(name='y', shape=[None, 4, 5])
                z = paddle.static.nn.fc(x, 100)
                loss = paddle.mean(z)
                scheduler = paddle.optimizer.lr.NoamDecay(d_model=0.01, warmup_steps=100, verbose=True)
                sgd = paddle.optimizer.SGD(learning_rate=scheduler)
                sgd.minimize(loss)

            exe = paddle.static.Executor()
            exe.run(start_prog)
            for epoch in range(20):
                for batch_id in range(5):
                    out = exe.run(
                        main_prog,
                        feed={
                            'x': np.random.randn(3, 4, 5).astype('float32'),
                            'y': np.random.randn(3, 4, 5).astype('float32')
                        },
                        fetch_list=loss.name)
                    scheduler.step()    # If you update learning rate each step
              # scheduler.step()        # If you update learning rate each epoch

    """

    def __init__(self,
                 d_model,
                 warmup_steps,
                 learning_rate=1.0,
                 last_epoch=-1,
                 verbose=False):
        self.d_model = d_model
        self.warmup_steps = warmup_steps
        super(NoamDecay, self).__init__(learning_rate, last_epoch, verbose)

    def get_lr(self):
        if self.last_epoch == 0:
            a = 1
        else:
            a = self.last_epoch**-0.5
        b = self.warmup_steps**-1.5 * self.last_epoch
        return self.base_lr * (self.d_model**-0.5) * min(a, b)


class PiecewiseDecay(LRScheduler):
    """

    Piecewise learning rate scheduler.

    The algorithm can be described as the code below:

    .. code-block:: text

        boundaries = [100, 200]
        values = [1.0, 0.5, 0.1]
        if epoch < 100:
            learning_rate = 1.0
        elif 100 <= global_step < 200:
            learning_rate = 0.5
        else:
            learning_rate = 0.1

    Args:
        boundaries(list|tuple): A list/tuple of steps numbers. The type of element in the list is python int.
        values(list|tuple): A list/tuple of learning rate values that will be picked during different epoch boundaries.
            The type of element in the list is python float.
        last_epoch (int, optional):  The index of last epoch. Can be set to restart training. Default: -1, means initial learning rate.
        verbose (bool, optional): If ``True``, prints a message to stdout for each update. Default: ``False`` .

    Returns:
        ``PiecewiseDecay`` instance to schedule learning rate.

    Examples:

        .. code-block:: python

            import paddle
            import numpy as np

            # train on default dynamic graph mode
            linear = paddle.nn.Linear(10, 10)
            scheduler = paddle.optimizer.lr.PiecewiseDecay(boundaries=[3, 6, 9], values=[0.1, 0.2, 0.3, 0.4], verbose=True)
            sgd = paddle.optimizer.SGD(learning_rate=scheduler, parameters=linear.parameters())
            for epoch in range(20):
                for batch_id in range(5):
                    x = paddle.uniform([10, 10])
                    out = linear(x)
                    loss = paddle.mean(out)
                    loss.backward()
                    sgd.step()
                    sgd.clear_gradients()
                    scheduler.step()    # If you update learning rate each step
              # scheduler.step()        # If you update learning rate each epoch

            # train on static graph mode
            paddle.enable_static()
            main_prog = paddle.static.Program()
            start_prog = paddle.static.Program()
            with paddle.static.program_guard(main_prog, start_prog):
                x = paddle.static.data(name='x', shape=[None, 4, 5])
                y = paddle.static.data(name='y', shape=[None, 4, 5])
                z = paddle.static.nn.fc(x, 100)
                loss = paddle.mean(z)
                scheduler = paddle.optimizer.lr.PiecewiseDecay(boundaries=[3, 6, 9], values=[0.1, 0.2, 0.3, 0.4], verbose=True)
                sgd = paddle.optimizer.SGD(learning_rate=scheduler)
                sgd.minimize(loss)

            exe = paddle.static.Executor()
            exe.run(start_prog)
            for epoch in range(20):
                for batch_id in range(5):
                    out = exe.run(
                        main_prog,
                        feed={
                            'x': np.random.randn(3, 4, 5).astype('float32'),
                            'y': np.random.randn(3, 4, 5).astype('float32')
                        },
                        fetch_list=loss.name)
                    scheduler.step()    # If you update learning rate each step
              # scheduler.step()        # If you update learning rate each epoch
    """

    def __init__(self, boundaries, values, last_epoch=-1, verbose=False):
        self.boundaries = boundaries
        self.values = values
        super(PiecewiseDecay, self).__init__(
            last_epoch=last_epoch, verbose=verbose)

    def get_lr(self):
        for i in range(len(self.boundaries)):
            if self.last_epoch < self.boundaries[i]:
                return self.values[i]
        return self.values[len(self.values) - 1]


class NaturalExpDecay(LRScheduler):
    r"""

    Applies natural exponential decay to the initial learning rate.

    The algorithm can be described as following:

    .. math::

        new\_learning\_rate = learning\_rate * e^{- gamma * epoch}

    Args:
        learning_rate (float): The initial learning rate. It is a python float number.
        gamma (float, optional): A Ratio to update the learning rate, should greater than 0.0 to make learning rate decay. Default: 0.1.
        last_epoch (int, optional):  The index of last epoch. Can be set to restart training. Default: -1, means initial learning rate.
        verbose (bool, optional): If ``True``, prints a message to stdout for each update. Default: ``False`` .

    Returns:
        ``NaturalExpDecay`` instance to schedule learning rate.

    Examples:

        .. code-block:: python

            import paddle
            import numpy as np

            # train on default dynamic graph mode
            linear = paddle.nn.Linear(10, 10)
            scheduler = paddle.optimizer.lr.NaturalExpDecay(learning_rate=0.5, gamma=0.1, verbose=True)
            sgd = paddle.optimizer.SGD(learning_rate=scheduler, parameters=linear.parameters())
            for epoch in range(20):
                for batch_id in range(5):
                    x = paddle.uniform([10, 10])
                    out = linear(x)
                    loss = paddle.mean(out)
                    loss.backward()
                    sgd.step()
                    sgd.clear_gradients()
                    scheduler.step()    # If you update learning rate each step
              # scheduler.step()        # If you update learning rate each epoch

            # train on static graph mode
            paddle.enable_static()
            main_prog = paddle.static.Program()
            start_prog = paddle.static.Program()
            with paddle.static.program_guard(main_prog, start_prog):
                x = paddle.static.data(name='x', shape=[None, 4, 5])
                y = paddle.static.data(name='y', shape=[None, 4, 5])
                z = paddle.static.nn.fc(x, 100)
                loss = paddle.mean(z)
                scheduler = paddle.optimizer.lr.NaturalExpDecay(learning_rate=0.5, gamma=0.1, verbose=True)
                sgd = paddle.optimizer.SGD(learning_rate=scheduler)
                sgd.minimize(loss)

            exe = paddle.static.Executor()
            exe.run(start_prog)
            for epoch in range(20):
                for batch_id in range(5):
                    out = exe.run(
                        main_prog,
                        feed={
                            'x': np.random.randn(3, 4, 5).astype('float32'),
                            'y': np.random.randn(3, 4, 5).astype('float32')
                        },
                        fetch_list=loss.name)
                    scheduler.step()    # If you update learning rate each step
              # scheduler.step()        # If you update learning rate each epoch
    """

    def __init__(self, learning_rate, gamma, last_epoch=-1, verbose=False):
        assert gamma > 0.0, " 'gamma' must be a positive number so that the learning rate will decay."
        self.gamma = gamma
        super(NaturalExpDecay, self).__init__(learning_rate, last_epoch,
                                              verbose)

    def get_lr(self):
        return self.base_lr * math.exp(-1 * self.gamma * self.last_epoch)


class InverseTimeDecay(LRScheduler):
    r"""

    Applies inverse time decay to the initial learning rate.

    The algorithm can be described as following:

    .. math::

        new\_learning\_rate = \frac{learning\_rate}{1 + gamma * epoch}

    Args:
        learning_rate (float): The initial learning rate. It is a python float number.
        gamma (float, optional): The Ratio that the learning rate will be reduced. ``new_lr = origin_lr * gamma`` .
            It should be less than 1.0. Default: 0.1.
        last_epoch (int, optional):  The index of last epoch. Can be set to restart training. Default: -1, means initial learning rate.
        verbose (bool, optional): If ``True``, prints a message to stdout for each update. Default: ``False`` .

    Returns:
        ``InverseTimeDecay`` instance to schedule learning rate.

    Examples:

        .. code-block:: python

            import paddle
            import numpy as np

            # train on default dynamic graph mode
            linear = paddle.nn.Linear(10, 10)
            scheduler = paddle.optimizer.lr.InverseTimeDecay(learning_rate=0.5, gamma=0.1, verbose=True)
            sgd = paddle.optimizer.SGD(learning_rate=scheduler, parameters=linear.parameters())
            for epoch in range(20):
                for batch_id in range(5):
                    x = paddle.uniform([10, 10])
                    out = linear(x)
                    loss = paddle.mean(out)
                    loss.backward()
                    sgd.step()
                    sgd.clear_gradients()
                    scheduler.step()    # If you update learning rate each step
              # scheduler.step()        # If you update learning rate each epoch

            # train on static graph mode
            paddle.enable_static()
            main_prog = paddle.static.Program()
            start_prog = paddle.static.Program()
            with paddle.static.program_guard(main_prog, start_prog):
                x = paddle.static.data(name='x', shape=[None, 4, 5])
                y = paddle.static.data(name='y', shape=[None, 4, 5])
                z = paddle.static.nn.fc(x, 100)
                loss = paddle.mean(z)
                scheduler = paddle.optimizer.lr.InverseTimeDecay(learning_rate=0.5, gamma=0.1, verbose=True)
                sgd = paddle.optimizer.SGD(learning_rate=scheduler)
                sgd.minimize(loss)

            exe = paddle.static.Executor()
            exe.run(start_prog)
            for epoch in range(20):
                for batch_id in range(5):
                    out = exe.run(
                        main_prog,
                        feed={
                            'x': np.random.randn(3, 4, 5).astype('float32'),
                            'y': np.random.randn(3, 4, 5).astype('float32')
                        },
                        fetch_list=loss.name)
                    scheduler.step()    # If you update learning rate each step
              # scheduler.step()        # If you update learning rate each epoch

    """

    def __init__(self, learning_rate, gamma, last_epoch=-1, verbose=False):
        self.gamma = gamma
        super(InverseTimeDecay, self).__init__(learning_rate, last_epoch,
                                               verbose)

    def get_lr(self):
        return self.base_lr / (1 + self.gamma * self.last_epoch)


class PolynomialDecay(LRScheduler):
    r"""

    Applies polynomial decay to the initial learning rate.

    The algorithm can be described as following.

    If cycle is set to True, then:

    .. math::

        decay\_steps & = decay\_steps * math.ceil(\frac{epoch}{decay\_steps})

        new\_learning\_rate & = (learning\_rate-end\_lr)*(1-\frac{epoch}{decay\_steps})^{power}+end\_lr

    If cycle is set to False, then:

    .. math::

        epoch & = min(epoch, decay\_steps)

        new\_learning\_rate & = (learning\_rate-end\_lr)*(1-\frac{epoch}{decay\_steps})^{power}+end\_lr


    Args:
        learning_rate (float): The initial learning rate. It is a python float number.
        decay_steps(int): The decay step size. It determines the decay cycle. It must be a positive integer.
        end_lr(float, optional): The minimum final learning rate. Default: 0.0001.
        power(float, optional): Power of polynomial, should greater than 0.0 to get learning rate decay. Default: 1.0.
        cycle(bool, optional): Whether the learning rate rises again. If True, then the learning rate will rise when it decrease
            to ``end_lr`` .  If False, the learning rate is monotone decreasing. Default: False.
        last_epoch (int, optional):  The index of last epoch. Can be set to restart training. Default: -1, means initial learning rate.
        verbose (bool, optional): If ``True``, prints a message to stdout for each update. Default: ``False`` .

    Returns:
        ``PolynomialDecay`` instance to schedule learning rate.

    Examples:

        .. code-block:: python

            import paddle
            import numpy as np

            # train on default dynamic graph mode
            linear = paddle.nn.Linear(10, 10)
            scheduler = paddle.optimizer.lr.PolynomialDecay(learning_rate=0.5, decay_steps=20, verbose=True)
            sgd = paddle.optimizer.SGD(learning_rate=scheduler, parameters=linear.parameters())
            for epoch in range(20):
                for batch_id in range(5):
                    x = paddle.uniform([10, 10])
                    out = linear(x)
                    loss = paddle.mean(out)
                    loss.backward()
                    sgd.step()
                    sgd.clear_gradients()
                    scheduler.step()    # If you update learning rate each step
              # scheduler.step()        # If you update learning rate each epoch

            # train on static graph mode
            paddle.enable_static()
            main_prog = paddle.static.Program()
            start_prog = paddle.static.Program()
            with paddle.static.program_guard(main_prog, start_prog):
                x = paddle.static.data(name='x', shape=[None, 4, 5])
                y = paddle.static.data(name='y', shape=[None, 4, 5])
                z = paddle.static.nn.fc(x, 100)
                loss = paddle.mean(z)
                scheduler = paddle.optimizer.lr.PolynomialDecay(learning_rate=0.5, decay_steps=20, verbose=True)
                sgd = paddle.optimizer.SGD(learning_rate=scheduler)
                sgd.minimize(loss)

            exe = paddle.static.Executor()
            exe.run(start_prog)
            for epoch in range(20):
                for batch_id in range(5):
                    out = exe.run(
                        main_prog,
                        feed={
                            'x': np.random.randn(3, 4, 5).astype('float32'),
                            'y': np.random.randn(3, 4, 5).astype('float32')
                        },
                        fetch_list=loss.name)
                    scheduler.step()    # If you update learning rate each step
              # scheduler.step()        # If you update learning rate each epoch
    """

    def __init__(self,
                 learning_rate,
                 decay_steps,
                 end_lr=0.0001,
                 power=1.0,
                 cycle=False,
                 last_epoch=-1,
                 verbose=False):
        assert decay_steps > 0 and isinstance(
            decay_steps, int), " 'decay_steps' must be a positive integer."
        self.decay_steps = decay_steps
        self.end_lr = end_lr
        assert power > 0.0, " 'power' must be greater than 0.0 so that the learning rate will decay."
        self.power = power
        self.cycle = cycle
        super(PolynomialDecay, self).__init__(learning_rate, last_epoch,
                                              verbose)

    def get_lr(self):
        tmp_epoch_num = self.last_epoch
        tmp_decay_steps = self.decay_steps
        if self.cycle:
            div_res = math.ceil(
                float(self.last_epoch) / float(self.decay_steps))

            if self.last_epoch == 0:
                div_res = 1
            tmp_decay_steps = self.decay_steps * div_res
        else:
            tmp_epoch_num = min(self.last_epoch, self.decay_steps)

        return (self.base_lr - self.end_lr) * (
            (1 - float(tmp_epoch_num) / float(tmp_decay_steps)
             )**self.power) + self.end_lr


class LinearWarmup(LRScheduler):
    r"""

    Linear learning rate warm up strategy. Update the learning rate preliminarily before the normal learning rate scheduler.
    For more information, please refer to `Bag of Tricks for Image Classification with Convolutional Neural Networks <https://arxiv.org/abs/1812.01187>`_

    When epoch < warmup_steps, learning rate is updated as:

    .. math::

            lr = start\_lr + (end\_lr - start\_lr) * \frac{epoch}{warmup\_steps}

    where start_lr is the initial learning rate, and end_lr is the final learning rate;

    When epoch >= warmup_steps, learning rate is updated as:

    .. math::

            lr = learning_rate

    where ``learning_rate`` is float or any subclass of ``LRScheduler`` .

    Args:
        learning_rate (float|LRScheduler): The learning rate after warm-up. It is a python float number or any subclass of ``LRScheduler`` .
        warmup_steps (int): total steps of warm up. It must be a positive integer.
        start_lr (float): Initial learning rate of warm up.
        end_lr (float): Final learning rate of warm up.
        last_epoch (int, optional):  The index of last epoch. Can be set to restart training. Default: -1, means initial learning rate.
        verbose (bool, optional): If ``True``, prints a message to stdout for each update. Default: ``False`` .

    Returns:
        ``LinearWarmup`` instance to schedule learning rate.

    Examples:

        .. code-block:: python

            import paddle
            import numpy as np

            # train on default dynamic graph mode
            linear = paddle.nn.Linear(10, 10)
            scheduler = paddle.optimizer.lr.LinearWarmup(
                    learning_rate=0.5, warmup_steps=20, start_lr=0, end_lr=0.5, verbose=True)
            sgd = paddle.optimizer.SGD(learning_rate=scheduler, parameters=linear.parameters())
            for epoch in range(20):
                for batch_id in range(5):
                    x = paddle.uniform([10, 10])
                    out = linear(x)
                    loss = paddle.mean(out)
                    loss.backward()
                    sgd.step()
                    sgd.clear_gradients()
                    scheduler.step()    # If you update learning rate each step
              # scheduler.step()        # If you update learning rate each epoch

            # train on static graph mode
            paddle.enable_static()
            main_prog = paddle.static.Program()
            start_prog = paddle.static.Program()
            with paddle.static.program_guard(main_prog, start_prog):
                x = paddle.static.data(name='x', shape=[None, 4, 5])
                y = paddle.static.data(name='y', shape=[None, 4, 5])
                z = paddle.static.nn.fc(x, 100)
                loss = paddle.mean(z)
                scheduler = paddle.optimizer.lr.LinearWarmup(
                    learning_rate=0.5, warmup_steps=20, start_lr=0, end_lr=0.5, verbose=True)
                sgd = paddle.optimizer.SGD(learning_rate=scheduler)
                sgd.minimize(loss)

            exe = paddle.static.Executor()
            exe.run(start_prog)
            for epoch in range(20):
                for batch_id in range(5):
                    out = exe.run(
                        main_prog,
                        feed={
                            'x': np.random.randn(3, 4, 5).astype('float32'),
                            'y': np.random.randn(3, 4, 5).astype('float32')
                        },
                        fetch_list=loss.name)
                    scheduler.step()    # If you update learning rate each step
              # scheduler.step()        # If you update learning rate each epoch
    """

    def __init__(self,
                 learning_rate,
                 warmup_steps,
                 start_lr,
                 end_lr,
                 last_epoch=-1,
                 verbose=False):
        type_check = isinstance(learning_rate, float) or isinstance(
            learning_rate, int) or isinstance(learning_rate, LRScheduler)
        if not type_check:
            raise TypeError(
                "the type of learning_rate should be [int, float or LRScheduler], the current type is {}".
                format(learning_rate))
        self.learning_rate = learning_rate
        assert warmup_steps > 0 and isinstance(
            warmup_steps, int), " 'warmup_steps' must be a positive integer."
        self.warmup_steps = warmup_steps
        self.start_lr = start_lr
        self.end_lr = end_lr
        assert end_lr > start_lr, "end_lr {} must be greater than start_lr {}".format(
            end_lr, start_lr)
        super(LinearWarmup, self).__init__(start_lr, last_epoch, verbose)

    def state_dict(self):
        """
        Returns the state of the LinearWarmup scheduler as a :class:`dict`.

        It is a subset of ``self.__dict__`` .
        """
        state_dict = super(LinearWarmup, self).state_dict()
        if isinstance(self.learning_rate, LRScheduler):
            state_dict["LinearWarmup_LR"] = self.learning_rate.state_dict()
        return state_dict

    def set_state_dict(self, state_dict):
        """
        Loads state_dict for LinearWarmup scheduler.
        """
        super(LinearWarmup, self).set_state_dict(state_dict)
        if isinstance(self.learning_rate, LRScheduler):
            self.learning_rate.set_state_dict(state_dict["LinearWarmup_LR"])

    def get_lr(self):
        if self.last_epoch < self.warmup_steps:
            return (self.end_lr - self.start_lr) * float(
                self.last_epoch) / float(self.warmup_steps) + self.start_lr
        else:
            if isinstance(self.learning_rate, LRScheduler):
                self.learning_rate.step(self.last_epoch - self.warmup_steps)
                return self.learning_rate()

            return self.learning_rate


class ExponentialDecay(LRScheduler):
    r"""

    Update learning rate by `gamma` each epoch.

    The algorithm can be described as following.

    .. math::

        new\_learning\_rate = last\_learning\_rate * gamma

    Args:
        learning_rate (float): The initial learning rate. It is a python float number.
        gamma (float): The Ratio that the learning rate will be reduced. ``new_lr = origin_lr * gamma`` .
            It should be in interval (0.0, 1.0).
        last_epoch (int, optional):  The index of last epoch. Can be set to restart training. Default: -1, means initial learning rate.
        verbose (bool, optional): If ``True``, prints a message to stdout for each update. Default: ``False`` .

    Returns:
        ``ExponentialDecay`` instance to schedule learning rate.

    Examples:

        .. code-block:: python

            import paddle
            import numpy as np

            # train on default dynamic graph mode
            linear = paddle.nn.Linear(10, 10)
            scheduler = paddle.optimizer.lr.ExponentialDecay(learning_rate=0.5, gamma=0.9, verbose=True)
            sgd = paddle.optimizer.SGD(learning_rate=scheduler, parameters=linear.parameters())
            for epoch in range(20):
                for batch_id in range(5):
                    x = paddle.uniform([10, 10])
                    out = linear(x)
                    loss = paddle.mean(out)
                    loss.backward()
                    sgd.step()
                    sgd.clear_gradients()
                    scheduler.step()    # If you update learning rate each step
              # scheduler.step()        # If you update learning rate each epoch

            # train on static graph mode
            paddle.enable_static()
            main_prog = paddle.static.Program()
            start_prog = paddle.static.Program()
            with paddle.static.program_guard(main_prog, start_prog):
                x = paddle.static.data(name='x', shape=[None, 4, 5])
                y = paddle.static.data(name='y', shape=[None, 4, 5])
                z = paddle.static.nn.fc(x, 100)
                loss = paddle.mean(z)
                scheduler = paddle.optimizer.lr.ExponentialDecay(learning_rate=0.5, gamma=0.9, verbose=True)
                sgd = paddle.optimizer.SGD(learning_rate=scheduler)
                sgd.minimize(loss)

            exe = paddle.static.Executor()
            exe.run(start_prog)
            for epoch in range(20):
                for batch_id in range(5):
                    out = exe.run(
                        main_prog,
                        feed={
                            'x': np.random.randn(3, 4, 5).astype('float32'),
                            'y': np.random.randn(3, 4, 5).astype('float32')
                        },
                        fetch_list=loss.name)
                    scheduler.step()    # If you update learning rate each step
              # scheduler.step()        # If you update learning rate each epoch
    """

    def __init__(self, learning_rate, gamma, last_epoch=-1, verbose=False):
        assert gamma > 0.0 and gamma < 1.0, " 'gamma' must be in interval (0.0, 1.0) so that the learning rate will decay."
        self.gamma = gamma
        super(ExponentialDecay, self).__init__(learning_rate, last_epoch,
                                               verbose)

    def get_lr(self):
        return self.base_lr * (self.gamma**self.last_epoch)


class MultiStepDecay(LRScheduler):
    """
    Update the learning rate by ``gamma`` once ``epoch`` reaches one of the milestones.

    The algorithm can be described as the code below.

    .. code-block:: text

        learning_rate = 0.5
        milestones = [30, 50]
        gamma = 0.1
        if epoch < 30:
            learning_rate = 0.5
        elif epoch < 50:
            learning_rate = 0.05
        else:
            learning_rate = 0.005

    Args:
        learning_rate (float): The initial learning rate. It is a python float number.
        milestones (tuple|list): List or tuple of each boundaries. Must be increasing.
        gamma (float, optional): The Ratio that the learning rate will be reduced. ``new_lr = origin_lr * gamma`` .
            It should be less than 1.0. Default: 0.1.
        last_epoch (int, optional):  The index of last epoch. Can be set to restart training. Default: -1, means initial learning rate.
        verbose (bool, optional): If ``True``, prints a message to stdout for each update. Default: ``False`` .


    Returns:
        ``MultiStepDecay`` instance to schedule learning rate.

    Examples:

        .. code-block:: python

            import paddle
            import numpy as np

            # train on default dynamic graph mode
            linear = paddle.nn.Linear(10, 10)
            scheduler = paddle.optimizer.lr.MultiStepDecay(learning_rate=0.5, milestones=[2, 4, 6], gamma=0.8, verbose=True)
            sgd = paddle.optimizer.SGD(learning_rate=scheduler, parameters=linear.parameters())
            for epoch in range(20):
                for batch_id in range(5):
                    x = paddle.uniform([10, 10])
                    out = linear(x)
                    loss = paddle.mean(out)
                    loss.backward()
                    sgd.step()
                    sgd.clear_gradients()
                    scheduler.step()    # If you update learning rate each step
              # scheduler.step()        # If you update learning rate each epoch

            # train on static graph mode
            paddle.enable_static()
            main_prog = paddle.static.Program()
            start_prog = paddle.static.Program()
            with paddle.static.program_guard(main_prog, start_prog):
                x = paddle.static.data(name='x', shape=[None, 4, 5])
                y = paddle.static.data(name='y', shape=[None, 4, 5])
                z = paddle.static.nn.fc(x, 100)
                loss = paddle.mean(z)
                scheduler = paddle.optimizer.lr.MultiStepDecay(learning_rate=0.5, milestones=[2, 4, 6], gamma=0.8, verbose=True)
                sgd = paddle.optimizer.SGD(learning_rate=scheduler)
                sgd.minimize(loss)

            exe = paddle.static.Executor()
            exe.run(start_prog)
            for epoch in range(20):
                for batch_id in range(5):
                    out = exe.run(
                        main_prog,
                        feed={
                            'x': np.random.randn(3, 4, 5).astype('float32'),
                            'y': np.random.randn(3, 4, 5).astype('float32')
                        },
                        fetch_list=loss.name)
                    scheduler.step()    # If you update learning rate each step
              # scheduler.step()        # If you update learning rate each epoch
    """

    def __init__(self,
                 learning_rate,
                 milestones,
                 gamma=0.1,
                 last_epoch=-1,
                 verbose=False):
        if not isinstance(milestones, (tuple, list)):
            raise TypeError(
                "The type of 'milestones' in 'MultiStepDecay' must be 'tuple, list', but received %s."
                % type(milestones))

        if not all([
                milestones[i] < milestones[i + 1]
                for i in range(len(milestones) - 1)
        ]):
            raise ValueError('The elements of milestones must be incremented')
        if gamma >= 1.0:
            raise ValueError('gamma should be < 1.0.')

        self.milestones = milestones
        self.gamma = gamma
        super(MultiStepDecay, self).__init__(learning_rate, last_epoch, verbose)

    def get_lr(self):
        for i in range(len(self.milestones)):
            if self.last_epoch < self.milestones[i]:
                return self.base_lr * (self.gamma**i)
        return self.base_lr * (self.gamma**len(self.milestones))


class StepDecay(LRScheduler):
    """
    Update the learning rate of ``optimizer`` by ``gamma`` every ``step_size`` number of epoch.

    The algorithm can be described as the code below.

    .. code-block:: text

        learning_rate = 0.5
        step_size = 30
        gamma = 0.1

        learning_rate = 0.5     if epoch < 30
        learning_rate = 0.05    if 30 <= epoch < 60
        learning_rate = 0.005   if 60 <= epoch < 90
        ...

    Args:
        learning_rate (float): The initial learning rate. It is a python float number.
        step_size (int): the interval to update. It must be a positive integer.
        gamma (float, optional): The Ratio that the learning rate will be reduced. ``new_lr = origin_lr * gamma`` .
            It should be less than 1.0. Default: 0.1.
        last_epoch (int, optional):  The index of last epoch. Can be set to restart training. Default: -1, means initial learning rate.
        verbose (bool, optional): If ``True``, prints a message to stdout for each update. Default: ``False`` .

    Returns:
        ``StepDecay`` instance to schedule learning rate.


    Examples:

        .. code-block:: python

            import paddle
            import numpy as np

            # train on default dynamic graph mode
            linear = paddle.nn.Linear(10, 10)
            scheduler = paddle.optimizer.lr.StepDecay(learning_rate=0.5, step_size=5, gamma=0.8, verbose=True)
            sgd = paddle.optimizer.SGD(learning_rate=scheduler, parameters=linear.parameters())
            for epoch in range(20):
                for batch_id in range(5):
                    x = paddle.uniform([10, 10])
                    out = linear(x)
                    loss = paddle.mean(out)
                    loss.backward()
                    sgd.step()
                    sgd.clear_gradients()
                    scheduler.step()    # If you update learning rate each step
              # scheduler.step()        # If you update learning rate each epoch

            # train on static graph mode
            paddle.enable_static()
            main_prog = paddle.static.Program()
            start_prog = paddle.static.Program()
            with paddle.static.program_guard(main_prog, start_prog):
                x = paddle.static.data(name='x', shape=[None, 4, 5])
                y = paddle.static.data(name='y', shape=[None, 4, 5])
                z = paddle.static.nn.fc(x, 100)
                loss = paddle.mean(z)
                scheduler = paddle.optimizer.lr.StepDecay(learning_rate=0.5, step_size=5, gamma=0.8, verbose=True)
                sgd = paddle.optimizer.SGD(learning_rate=scheduler)
                sgd.minimize(loss)

            exe = paddle.static.Executor()
            exe.run(start_prog)
            for epoch in range(20):
                for batch_id in range(5):
                    out = exe.run(
                        main_prog,
                        feed={
                            'x': np.random.randn(3, 4, 5).astype('float32'),
                            'y': np.random.randn(3, 4, 5).astype('float32')
                        },
                        fetch_list=loss.name)
                    scheduler.step()    # If you update learning rate each step
              # scheduler.step()        # If you update learning rate each epoch
    """

    def __init__(self,
                 learning_rate,
                 step_size,
                 gamma=0.1,
                 last_epoch=-1,
                 verbose=False):
        if not isinstance(step_size, int):
            raise TypeError(
                "The type of 'step_size' must be 'int', but received %s." %
                type(step_size))
        if gamma >= 1.0:
            raise ValueError('gamma should be < 1.0.')

        assert step_size > 0 and isinstance(
            step_size, int), " 'step_size' must be a positive integer."
        self.step_size = step_size
        self.gamma = gamma
        super(StepDecay, self).__init__(learning_rate, last_epoch, verbose)

    def get_lr(self):
        i = self.last_epoch // self.step_size
        return self.base_lr * (self.gamma**i)


class LambdaDecay(LRScheduler):
    """
    Sets the learning rate of ``optimizer`` by function ``lr_lambda`` . ``lr_lambda`` is funciton which receives ``epoch`` .

    The algorithm can be described as the code below.

    .. code-block:: text

        learning_rate = 0.5        # init learning_rate
        lr_lambda = lambda epoch: 0.95 ** epoch

        learning_rate = 0.5        # epoch 0, 0.5*0.95**0
        learning_rate = 0.475      # epoch 1, 0.5*0.95**1
        learning_rate = 0.45125    # epoch 2, 0.5*0.95**2

    Args:
        learning_rate (float): The initial learning rate. It is a python float number.
        lr_lambda (function): A function which computes a factor by ``epoch`` , and then multiply the initial learning rate by this factor.
        last_epoch (int, optional):  The index of last epoch. Can be set to restart training. Default: -1, means initial learning rate.
        verbose (bool, optional): If ``True``, prints a message to stdout for each update. Default: ``False`` .

    Returns:
        ``LambdaDecay`` instance to schedule learning rate.

    Examples:

        .. code-block:: python

            import paddle
            import numpy as np

            # train on default dynamic graph mode
            linear = paddle.nn.Linear(10, 10)
            scheduler = paddle.optimizer.lr.LambdaDecay(learning_rate=0.5, lr_lambda=lambda x:0.95**x, verbose=True)
            sgd = paddle.optimizer.SGD(learning_rate=scheduler, parameters=linear.parameters())
            for epoch in range(20):
                for batch_id in range(5):
                    x = paddle.uniform([10, 10])
                    out = linear(x)
                    loss = paddle.mean(out)
                    loss.backward()
                    sgd.step()
                    sgd.clear_gradients()
                    scheduler.step()    # If you update learning rate each step
              # scheduler.step()        # If you update learning rate each epoch

            # train on static graph mode
            paddle.enable_static()
            main_prog = paddle.static.Program()
            start_prog = paddle.static.Program()
            with paddle.static.program_guard(main_prog, start_prog):
                x = paddle.static.data(name='x', shape=[None, 4, 5])
                y = paddle.static.data(name='y', shape=[None, 4, 5])
                z = paddle.static.nn.fc(x, 100)
                loss = paddle.mean(z)
                scheduler = paddle.optimizer.lr.LambdaDecay(learning_rate=0.5, lr_lambda=lambda x:0.95**x, verbose=True)
                sgd = paddle.optimizer.SGD(learning_rate=scheduler)
                sgd.minimize(loss)

            exe = paddle.static.Executor()
            exe.run(start_prog)
            for epoch in range(20):
                for batch_id in range(5):
                    out = exe.run(
                        main_prog,
                        feed={
                            'x': np.random.randn(3, 4, 5).astype('float32'),
                            'y': np.random.randn(3, 4, 5).astype('float32')
                        },
                        fetch_list=loss.name)
                    scheduler.step()    # If you update learning rate each step
              # scheduler.step()        # If you update learning rate each epoch

    """

    def __init__(self, learning_rate, lr_lambda, last_epoch=-1, verbose=False):
        if not callable(lr_lambda):
            raise TypeError(
                "The type of 'lr_lambda' in 'LambdaDecay' must be 'function', but received %s."
                % type(lr_lambda))

        self.lr_lambda = lr_lambda
        super(LambdaDecay, self).__init__(learning_rate, last_epoch, verbose)

    def get_lr(self):
        return self.base_lr * self.lr_lambda(self.last_epoch)


class ReduceOnPlateau(LRScheduler):
    """
    Reduce learning rate when ``metrics`` has stopped descending. Models often benefit from reducing the learning rate
    by 2 to 10 times once model performance has no longer improvement.

    The ``metrics`` is the one which has been pass into ``step`` , it must be 1-D Tensor with shape [1]. When ``metrics``
    stop descending for a ``patience`` number of epochs, the learning rate will be reduced to ``learning_rate * factor`` .
    (Specially, ``mode`` can also be set to ``'max`` , in this case, when ``metrics`` stop ascending for a ``patience``
    number of epochs, the learning rate will be reduced.)

    In addition, After each reduction, it will wait a ``cooldown`` number of epochs before resuming above operation.

    Args:
        learning_rate (float): The initial learning rate. It is a python float number.
        mode (str, optional): ``'min'`` or ``'max'`` can be selected. Normally, it is ``'min'`` , which means that the
            learning rate will reduce when ``loss`` stops descending. Specially, if it's set to ``'max'`` ,  the learning
            rate will reduce when ``loss`` stops ascending. Default: ``'min'`` .
        factor (float, optional): The Ratio that the learning rate will be reduced. ``new_lr = origin_lr * factor`` .
            It should be less than 1.0. Default: 0.1.
        patience (int, optional): When ``loss`` doesn't improve for this number of epochs, learing rate will be reduced.
            Default: 10.
        threshold (float, optional): ``threshold`` and ``threshold_mode`` will determine the minimum change of ``loss`` .
            This make tiny changes of ``loss`` will be ignored. Default: 1e-4.
        threshold_mode (str, optional): ``'rel'`` or ``'abs'`` can be selected. In ``'rel'`` mode, the minimum change of ``loss``
            is ``last_loss * threshold`` , where ``last_loss`` is ``loss`` in last epoch. In ``'abs'`` mode, the minimum
            change of ``loss`` is ``threshold`` . Default: ``'rel'`` .
        cooldown (int, optional): The number of epochs to wait before resuming normal operation. Default: 0.
        min_lr (float, optional): The lower bound of the learning rate after reduction. Default: 0.
        epsilon (float, optional): Minimal decay applied to lr. If the difference between new and old lr is smaller than epsilon,
            the update is ignored. Default: 1e-8.
        verbose (bool, optional): If ``True``, prints a message to stdout for each update. Default: ``False``.


    Returns:
        ``ReduceOnPlateau`` instance to schedule learning rate.


    Examples:
        .. code-block:: python

            import paddle
            import numpy as np

            # train on default dynamic graph mode
            linear = paddle.nn.Linear(10, 10)
            scheduler = paddle.optimizer.lr.ReduceOnPlateau(learning_rate=1.0, factor=0.5, patience=5, verbose=True)
            sgd = paddle.optimizer.SGD(learning_rate=scheduler, parameters=linear.parameters())
            for epoch in range(20):
                for batch_id in range(5):
                    x = paddle.uniform([10, 10])
                    out = linear(x)
                    loss = paddle.mean(out)
                    loss.backward()
                    sgd.step()
                    sgd.clear_gradients()
                    scheduler.step(loss)    # If you update learning rate each step
              # scheduler.step(loss)        # If you update learning rate each epoch

            # train on static graph mode
            paddle.enable_static()
            main_prog = paddle.static.Program()
            start_prog = paddle.static.Program()
            with paddle.static.program_guard(main_prog, start_prog):
                x = paddle.static.data(name='x', shape=[None, 4, 5])
                y = paddle.static.data(name='y', shape=[None, 4, 5])
                z = paddle.static.nn.fc(x, 100)
                loss = paddle.mean(z)
                scheduler = paddle.optimizer.lr.ReduceOnPlateau(learning_rate=1.0, factor=0.5, patience=5, verbose=True)
                sgd = paddle.optimizer.SGD(learning_rate=scheduler)
                sgd.minimize(loss)

            exe = paddle.static.Executor()
            exe.run(start_prog)
            for epoch in range(20):
                for batch_id in range(5):
                    out = exe.run(
                        main_prog,
                        feed={
                            'x': np.random.randn(3, 4, 5).astype('float32'),
                            'y': np.random.randn(3, 4, 5).astype('float32')
                        },
                        fetch_list=loss.name)
                    scheduler.step(out[0])    # If you update learning rate each step
              # scheduler.step(out[0])        # If you update learning rate each epoch

    """

    def __init__(self,
                 learning_rate,
                 mode='min',
                 factor=0.1,
                 patience=10,
                 threshold=1e-4,
                 threshold_mode='rel',
                 cooldown=0,
                 min_lr=0,
                 epsilon=1e-8,
                 verbose=False):
        mode = mode.lower()
        if mode not in ['min', 'max']:
            raise ValueError('mode: ' + mode + ' is unknown!')
        self.mode = mode

        if factor >= 1.0:
            raise ValueError(
                'new_lr = origin_lr * gamma and gamma should be < 1.0.')
        self.factor = factor

        threshold_mode = threshold_mode.lower()
        if threshold_mode not in ['rel', 'abs']:
            raise ValueError('threshold mode: ' + threshold_mode +
                             ' is unknown!')
        self.threshold_mode = threshold_mode
        if not isinstance(learning_rate, (float, int)):
            raise TypeError(
                "The type of 'learning_rate' in 'ReduceOnPlateau' must be 'float', but received %s."
                % type(learning_rate))

        self.patience = patience
        self.threshold = threshold
        self.threshold_mode = threshold_mode
        self.cooldown = cooldown
        self.min_lr = min_lr
        self.epsilon = epsilon

        self.cooldown_counter = 0
        self.best = None
        self.num_bad_epochs = 0

        # Can not call Parent __init__, so implement here.
        self.base_lr = float(learning_rate)
        self.last_lr = float(learning_rate)
        self.last_epoch = 0
        self.verbose = verbose
        self._var_name = None

    # "cooldown_counter / best / num_bad_epochs / last_epoch / last_lr" will be stored.
    def state_keys(self):
        self.keys = [
            'cooldown_counter', 'best', 'num_bad_epochs', 'last_epoch',
            'last_lr'
        ]

    def step(self, metrics, epoch=None):
        """
        step should be called after `optimizer.step()` . It will update the learning rate in optimizer according to ``metrics`` .
        The new learning rate will take effect on next epoch.

        Args:
            metrics (Tensor|numpy.ndarray|float): Which will be monitored to determine whether the learning rate will reduce.
                If it stop descending for a ``patience`` number of epochs, the learning rate will reduce. If it's 'Tensor' or
                'numpy.ndarray', its shape must be [1].
            epoch (int, None): specify current epoch. Default: None. Auto-increment from last_epoch=-1.

        Returns:
            None

        Examples:
            Please refer to the example of current LRScheduler.
        """
        if epoch is None:
            self.last_epoch = self.last_epoch + 1
        else:
            self.last_epoch = epoch

        if not _in_legacy_dygraph():
            tmp = core.eager.Tensor
        else:
            # need to declarate explicitly
            from paddle.framework import VarBase as Tensor
            tmp = Tensor
        # loss must be float, numpy.ndarray or 1-D Tensor with shape [1]
        if isinstance(metrics, (tmp, numpy.ndarray)):
            assert len(metrics.shape) == 1 and metrics.shape[0] == 1, "the metrics.shape " \
                                                                      "should be (1L,), but the current metrics.shape is {}. Maybe that " \
                                                                      "you should call paddle.mean to process it first.".format(
                metrics.shape)
        elif not isinstance(metrics,
                            (int, float, numpy.float32, numpy.float64)):
            raise TypeError(
                "metrics must be 'int', 'float', 'np.float', 'numpy.ndarray' or 'paddle.Tensor', but receive {}".
                format(type(metrics)))

        if self.cooldown_counter > 0:
            self.cooldown_counter -= 1
        else:
            if self.best is None or self._is_better(metrics, self.best):
                self.best = metrics
                self.num_bad_epochs = 0
            else:
                self.num_bad_epochs += 1

            if self.num_bad_epochs > self.patience:
                self.cooldown_counter = self.cooldown
                self.num_bad_epochs = 0
                new_lr = max(self.last_lr * self.factor, self.min_lr)
                if self.last_lr - new_lr > self.epsilon:
                    self.last_lr = new_lr
                    if self.verbose:
                        print('Epoch {}: {} set learning rate to {}.'.format(
                            self.last_epoch, self.__class__.__name__,
                            self.last_lr))

    def _is_better(self, current, best):
        if self.mode == 'min' and self.threshold_mode == 'rel':
            return current < best - best * self.threshold

        elif self.mode == 'min' and self.threshold_mode == 'abs':
            return current < best - self.threshold

        elif self.mode == 'max' and self.threshold_mode == 'rel':
            return current > best + best * self.threshold

        else:
            return current > best + self.threshold


class CosineAnnealingDecay(LRScheduler):
    r"""

    Set the learning rate using a cosine annealing schedule, where :math:`\eta_{max}` is set to
    the initial learning_rate. :math:`T_{cur}` is the number of epochs since the last restart in
    SGDR.

    The algorithm can be described as following.

    .. math::

        \eta_t & = \eta_{min} + \frac{1}{2}(\eta_{max} - \eta_{min})\left(1
        + \cos\left(\frac{T_{cur}}{T_{max}}\pi\right)\right),
        & T_{cur} \neq (2k+1)T_{max};

        \eta_{t+1} & = \eta_{t} + \frac{1}{2}(\eta_{max} - \eta_{min})
        \left(1 - \cos\left(\frac{1}{T_{max}}\pi\right)\right),
        & T_{cur} = (2k+1)T_{max}.

    It has been proposed in `SGDR: Stochastic Gradient Descent with Warm Restarts <https://arxiv.org/abs/1608.03983>`_.
    Note that this only implements the cosine annealing part of SGDR, and not the restarts.

    Args:
        learning_rate (float): The initial learning rate, that is :math:`\eta_{max}` . It can be set to python float or int number.
        T_max (int): Maximum number of iterations. It is half of the decay cycle of learning rate. It must be a positive integer.
        eta_min (float|int, optional): Minimum learning rate, that is :math:`\eta_{min}` . Default: 0.
        last_epoch (int, optional):  The index of last epoch. Can be set to restart training. Default: -1, means initial learning rate.
        verbose (bool, optional): If ``True``, prints a message to stdout for each update. Default: ``False`` .

    Returns:
        ``CosineAnnealingDecay`` instance to schedule learning rate.

    Examples:

        .. code-block:: python

            import paddle
            import numpy as np

            # train on default dynamic graph mode
            linear = paddle.nn.Linear(10, 10)
            scheduler = paddle.optimizer.lr.CosineAnnealingDecay(learning_rate=0.5, T_max=10, verbose=True)
            sgd = paddle.optimizer.SGD(learning_rate=scheduler, parameters=linear.parameters())
            for epoch in range(20):
                for batch_id in range(5):
                    x = paddle.uniform([10, 10])
                    out = linear(x)
                    loss = paddle.mean(out)
                    loss.backward()
                    sgd.step()
                    sgd.clear_gradients()
                    scheduler.step()    # If you update learning rate each step
              # scheduler.step()        # If you update learning rate each epoch

            # train on static graph mode
            paddle.enable_static()
            main_prog = paddle.static.Program()
            start_prog = paddle.static.Program()
            with paddle.static.program_guard(main_prog, start_prog):
                x = paddle.static.data(name='x', shape=[None, 4, 5])
                y = paddle.static.data(name='y', shape=[None, 4, 5])
                z = paddle.static.nn.fc(x, 100)
                loss = paddle.mean(z)
                scheduler = paddle.optimizer.lr.CosineAnnealingDecay(learning_rate=0.5, T_max=10, verbose=True)
                sgd = paddle.optimizer.SGD(learning_rate=scheduler)
                sgd.minimize(loss)

            exe = paddle.static.Executor()
            exe.run(start_prog)
            for epoch in range(20):
                for batch_id in range(5):
                    out = exe.run(
                        main_prog,
                        feed={
                            'x': np.random.randn(3, 4, 5).astype('float32'),
                            'y': np.random.randn(3, 4, 5).astype('float32')
                        },
                        fetch_list=loss.name)
                    scheduler.step()    # If you update learning rate each step
              # scheduler.step()        # If you update learning rate each epoch
    """

    def __init__(self,
                 learning_rate,
                 T_max,
                 eta_min=0,
                 last_epoch=-1,
                 verbose=False):
        if not isinstance(T_max, int):
            raise TypeError(
                "The type of 'T_max' in 'CosineAnnealingDecay' must be 'int', but received %s."
                % type(T_max))
        if not isinstance(eta_min, (float, int)):
            raise TypeError(
                "The type of 'eta_min' in 'CosineAnnealingDecay' must be 'float, int', but received %s."
                % type(eta_min))
        assert T_max > 0 and isinstance(
            T_max, int), " 'T_max' must be a positive integer."
        self.T_max = T_max
        self.eta_min = float(eta_min)
        super(CosineAnnealingDecay, self).__init__(learning_rate, last_epoch,
                                                   verbose)

    def get_lr(self):
        if self.last_epoch == 0:
            return self.base_lr
        elif (self.last_epoch - 1 - self.T_max) % (2 * self.T_max) == 0:
            return self.last_lr + (self.base_lr - self.eta_min) * (1 - math.cos(
                math.pi / self.T_max)) / 2

        return (1 + math.cos(math.pi * self.last_epoch / self.T_max)) / (
            1 + math.cos(math.pi * (self.last_epoch - 1) / self.T_max)) * (
                self.last_lr - self.eta_min) + self.eta_min

    def _get_closed_form_lr(self):
        return self.eta_min + (self.base_lr - self.eta_min) * (1 + math.cos(
            math.pi * self.last_epoch / self.T_max)) / 2


class MultiplicativeDecay(LRScheduler):
    """
    Multiply the learning rate of ``optimizer`` by the factor given in function ``lr_lambda`` .

    The algorithm can be described as the code below.

    .. code-block:: text

        learning_rate = 0.5        # init learning_rate
        lr_lambda = lambda epoch: 0.95

        learning_rate = 0.5        # epoch 0,
        learning_rate = 0.475      # epoch 1, 0.5*0.95
        learning_rate = 0.45125    # epoch 2, 0.475*0.95

    Args:
        learning_rate (float): The initial learning rate. It is a python float number.
        lr_lambda (function): A function which computes a factor by ``epoch`` , and then multiply the last learning rate by this factor.
        last_epoch (int, optional):  The index of last epoch. Can be set to restart training. Default: -1, means initial learning rate.
        verbose (bool, optional): If ``True``, prints a message to stdout for each update. Default: ``False`` .

    Returns:
        ``MultiplicativeDecay`` instance to schedule learning rate.

    Examples:

        .. code-block:: python

            import paddle
            import numpy as np

            # train on default dynamic graph mode
            linear = paddle.nn.Linear(10, 10)
            scheduler = paddle.optimizer.lr.MultiplicativeDecay(learning_rate=0.5, lr_lambda=lambda x:0.95, verbose=True)
            sgd = paddle.optimizer.SGD(learning_rate=scheduler, parameters=linear.parameters())
            for epoch in range(20):
                for batch_id in range(5):
                    x = paddle.uniform([10, 10])
                    out = linear(x)
                    loss = paddle.mean(out)
                    loss.backward()
                    sgd.step()
                    sgd.clear_gradients()
                    scheduler.step()    # If you update learning rate each step
              # scheduler.step()        # If you update learning rate each epoch

    """

    def __init__(self, learning_rate, lr_lambda, last_epoch=-1, verbose=False):
        if not callable(lr_lambda):
            raise TypeError(
                "The type of 'lr_lambda' in 'MultiplicativeDecay' must be 'function', but received %s."
                % type(lr_lambda))

        self.lr_lambda = lr_lambda
        super(MultiplicativeDecay, self).__init__(learning_rate, last_epoch,
                                                  verbose)

    def get_lr(self):
        cur_lr = self.base_lr
        for epoch in range(1, self.last_epoch + 1):
            cur_lr = cur_lr * self.lr_lambda(epoch)
        return cur_lr


class OneCycleLR(LRScheduler):
    r"""
    Sets the learning rate according to the one cycle learning rate scheduler.
    The scheduler adjusts the learning rate from an initial learning rate to the maximum learning rate and then
    from that maximum learning rate to the minimum learning rate, which is much less than the initial learning rate.

    It has been proposed in `Super-Convergence: Very Fast Training of Neural Networks Using Large Learning Rates <https://arxiv.org/abs/1708.07120>`_.

    Please note that the default behaviour of this scheduler follows the fastai implementation of one cycle,
    which claims that “unpublished work has shown even better results by using only two phases”.
    Set ``three_phase=True``, if you want the behaviour of this scheduler to be consistent with the paper.

    Also note that you should update learning rate each step.

    This implementation was adapted from `there <https://github.com/pytorch/pytorch/blob/e5ee6f5cf714812283ff4e49362fbdf37fbd8ea9/torch/optim/lr_scheduler.py#L1346>`_.

    Args:
        max_learning_rate (float): Upper boundary of learning rate during training.
             Functionally, it defines the initial learning rate and the minimum learning rate by ``divide_factor`` and
             ``final_divide_factor`` respectively.
        total_steps (int, optional): Number of total training steps.
            Note that one of total_steps and (epochs, steps_per_epoch) must be specified.
            If ``total_steps`` is not specified, it will be determined by ``epochs`` and ``steps_per_epoch``. Default: None.
        epochs (int, optional): Number of total training epochs. Default: None.
        steps_per_epoch (int, optional): Number of training steps for each epoch. Default: None.
        pct_start (float): The percentage of total steps, which used to increasing learning rate. Default: 0.3.
        anneal_strategy (str, optional): Strategy of adjusting learning rate.'cos' for cosine annealing,
            'linear' for linear annealing. Default: 'cos'.
        divide_factor (float, optional): Initial learning rate will be determined by initial_lr = max_lr/div_factor. Default: 25.
        final_divide_factor (float, optional): Minimum learning rate will be determined by minimum = max_lr/final_divide_factor. Default: 1e4.
        three_phase (bool, optional): Whether to use three phase.
            If ``True``:
                1. The learning rate will first increase from initial learning rate to maximum learning rate.
                2. Then it will decrease to initial learning rate. Number of step in this phase is the same as the one in first phase.
                3. Finally, it will decrease to minimum learning rate which is much less than initial learning rate.
            If ``False``:
                1. The learning rate will increase to maximum learning rate.
                2. Then it will directly decrease to minimum learning rate.
        last_epoch (int, optional):  The index of last epoch. Can be set to restart training. Default: -1, means initial learning rate.
        verbose (bool, optional): If ``True``, prints a message to stdout for each update. Default: ``False`` .

    Returns:
        ``OneCycleLR`` instance to schedule learning rate.

    Examples:
        .. code-block:: python
            import paddle
            import numpy as np
            # train on default dynamic graph mode
            linear = paddle.nn.Linear(10, 10)
            scheduler = paddle.optimizer.lr.OneCycleLR(max_learning_rate=1.0, total_steps=100, verbose=True)
            sgd = paddle.optimizer.SGD(learning_rate=scheduler, parameters=linear.parameters())
            for epoch in range(5):
                for batch_id in range(20):
                    x = paddle.uniform([10, 10])
                    out = linear(x)
                    loss = paddle.mean(out)
                    loss.backward()
                    sgd.step()
                    sgd.clear_gradients()
                    scheduler.step()        # You should update learning rate each step
            # train on static graph mode
            paddle.enable_static()
            main_prog = paddle.static.Program()
            start_prog = paddle.static.Program()
            with paddle.static.program_guard(main_prog, start_prog):
                x = paddle.static.data(name='x', shape=[None, 4, 5])
                y = paddle.static.data(name='y', shape=[None, 4, 5])
                z = paddle.static.nn.fc(x, 100)
                loss = paddle.mean(z)
                scheduler = paddle.optimizer.lr.OneCycleLR(max_learning_rate=1.0, total_steps=100, verbose=True)
                sgd = paddle.optimizer.SGD(learning_rate=scheduler)
                sgd.minimize(loss)
            exe = paddle.static.Executor()
            exe.run(start_prog)
            for epoch in range(5):
                for batch_id in range(20):
                    out = exe.run(
                        main_prog,
                        feed={
                            'x': np.random.randn(3, 4, 5).astype('float32'),
                            'y': np.random.randn(3, 4, 5).astype('float32')
                        },
                        fetch_list=loss.name)
                    scheduler.step()    # You should update learning rate each step
    """

    def __init__(self,
                 max_learning_rate,
                 total_steps=None,
                 epochs=None,
                 steps_per_epoch=None,
                 pct_start=0.3,
                 anneal_strategy='cos',
                 divide_factor=25.,
                 final_divide_factor=1e4,
                 three_phase=False,
                 last_epoch=-1,
                 verbose=False):
        # Check type of max_learning_rate
        if not isinstance(max_learning_rate, (float, int)):
            raise TypeError(
                "The type of learning rate must be float, but received {}".
                format(type(max_learning_rate)))
        # Check type and value of total_steps
        if total_steps is None and epochs is None and steps_per_epoch is None:
            raise ValueError(
                "either total_steps or (epochs, steps_per_epoch) must be specified"
            )
        elif total_steps is not None:
            if not isinstance(total_steps, int):
                raise TypeError("'total_step' must be 'int', but received {}".
                                format(type(total_steps)))
            if total_steps <= 0:
                raise ValueError("'total_step' must be a positive integer.")
            self.total_steps = total_steps
        else:
            # Check type and value of epochs and steps_per_epochs
            if not isinstance(epochs, int):
                raise TypeError("'epochs' must be 'int', but received {}".
                                format(type(epochs)))
            if not isinstance(steps_per_epoch, int):
                raise TypeError(
                    "'steps_per_epoch', must be 'int', but received {}".format(
                        type(steps_per_epoch)))
            if epochs < 0:
                raise ValueError("'epochs' must be a positive integer.")
            if steps_per_epoch < 0:
                raise ValueError(
                    "'steps_per_epoch' must be a positive integer.")
            self.total_steps = epochs * steps_per_epoch
        # Check type and value of pac_start
        if not isinstance(pct_start, float):
            raise TypeError("'pct_start' must be 'float', but received {}".
                            format(type(pct_start)))
        if pct_start < 0 or pct_start > 1:
            raise ValueError(
                "'pct_start' must be between 0 and 1, but received {}".format(
                    pct_start))

        max_lr = max_learning_rate
        initial_lr = max_lr / divide_factor
        min_lr = initial_lr / final_divide_factor

        if three_phase:
            self._end_steps = [
                float(pct_start * self.total_steps) - 1,
                float(2 * pct_start * self.total_steps) - 2,
                self.total_steps - 1
            ]
            self._schedule_phases = [
                {
                    'start_lr': initial_lr,
                    'end_lr': max_lr,
                },
                {
                    'start_lr': max_lr,
                    'end_lr': initial_lr,
                },
                {
                    'start_lr': initial_lr,
                    'end_lr': min_lr,
                },
            ]
        else:
            self._end_steps = [
                float(pct_start * self.total_steps) - 1, self.total_steps - 1
            ]
            self._schedule_phases = [
                {
                    'start_lr': initial_lr,
                    'end_lr': max_lr,
                },
                {
                    'start_lr': max_lr,
                    'end_lr': min_lr,
                },
            ]

        # Validate anneal_strategy
        if anneal_strategy not in ['cos', 'linear']:
            raise ValueError(
                "'anneal_strategy' must by one of 'cos' or 'linear', but received {}".
                format(anneal_strategy))
        elif anneal_strategy == 'cos':
            self.anneal_func = self._annealing_cos
        elif anneal_strategy == 'linear':
            self.anneal_func = self._annealing_linear

        super(OneCycleLR, self).__init__(initial_lr, last_epoch, verbose)

    def _annealing_cos(self, start, end, pct):
        cos_out = math.cos(math.pi * pct) + 1
        return end + (start - end) / 2.0 * cos_out

    def _annealing_linear(self, start, end, pct):
        return (end - start) * pct + start

    def get_lr(self):
        step_num = self.last_epoch

        if step_num > self.total_steps:
            raise ValueError(
                "Tried to step {} times. The specified number of total steps is {}"
                .format(step_num + 1, self.total_steps))

        start_step = 0
        for i, phase in enumerate(self._schedule_phases):
            end_step = self._end_steps[i]
            if step_num <= end_step or i == len(self._schedule_phases) - 1:
                pct = (step_num - start_step) / (end_step - start_step)
                computed_lr = self.anneal_func(phase['start_lr'],
                                               phase['end_lr'], pct)
                break
            start_step = end_step

        return computed_lr