precision_plugin.py

# Copyright The PyTorch Lightning team.
#
# 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 contextlib
from functools import partial
from typing import Any, Callable, Generator, List, Optional, Tuple, Union

import torch
from torch import Tensor
from torch.nn import Module
from torch.optim import Optimizer

import pytorch_lightning as pl
from pytorch_lightning.core.hooks import CheckpointHooks
from pytorch_lightning.utilities import grad_norm, GradClipAlgorithmType
from pytorch_lightning.utilities.types import _PARAMETERS


class PrecisionPlugin(CheckpointHooks):
    """Base class for all plugins handling the precision-specific parts of the training.

    The class attribute precision must be overwritten in child classes. The default value reflects fp32 training.
    """

    precision: Union[str, int] = 32

    def main_params(self, optimizer: Optimizer) -> _PARAMETERS:
        """The main params of the model.

        Returns the plain model params here. Maybe different in other precision plugins.
        """
        for group in optimizer.param_groups:
            yield from group["params"]

    def connect(
        self, model: Module, optimizers: List[Optimizer], lr_schedulers: List[Any]
    ) -> Tuple[Module, List[Optimizer], List[Any]]:
        """Connects this plugin to the accelerator and the training process."""
        return model, optimizers, lr_schedulers

    def pre_backward(self, model: "pl.LightningModule", closure_loss: Tensor) -> Tensor:
        """Run before precision plugin executes backward.

        Args:
            model: the model to be optimized
            closure_loss: the loss value obtained from the closure
        """
        model.trainer._call_callback_hooks("on_before_backward", closure_loss)
        model.trainer._call_lightning_module_hook("on_before_backward", closure_loss)
        return closure_loss

    def backward(
        self,
        model: "pl.LightningModule",
        closure_loss: Tensor,
        optimizer: Optional[Optimizer],
        *args: Any,
        **kwargs: Any,
    ) -> None:
        """Performs the actual backpropagation.

        Args:
            model: the model to be optimized
            closure_loss: the loss value obtained from the closure
            optimizer: current optimizer being used. ``None`` if using manual optimization
        """
        # do backward pass
        if model is not None and isinstance(model, pl.LightningModule):
            model.backward(closure_loss, optimizer, *args, **kwargs)
        else:
            self._run_backward(closure_loss, *args, **kwargs)

    def post_backward(self, model: "pl.LightningModule", closure_loss: Tensor) -> Tensor:
        """Run after precision plugin executes backward.

        Args:
            model: the model to be optimized
            closure_loss: the loss value obtained from the closure
        """
        # once backward has been applied, release graph
        closure_loss = closure_loss.detach()
        model.trainer._call_callback_hooks("on_after_backward")
        model.trainer._call_lightning_module_hook("on_after_backward")
        return closure_loss

    def _run_backward(self, tensor: Tensor, model: Optional[Module], *args: Any, **kwargs: Any) -> None:
        """Lightning-independent backward logic.

        Currently only used by Lightning Lite. Subject to further refactors.
        """
        tensor.backward(*args, **kwargs)

    def _after_closure(
        self, model: Union["pl.LightningModule", Module], optimizer: Optimizer, optimizer_idx: int
    ) -> None:
        """Utility to share some code after the closure has been run."""
        if not isinstance(model, pl.LightningModule):
            # none of this applies to Lite
            return
        trainer = model.trainer
        assert trainer is not None
        trainer._call_callback_hooks("on_before_optimizer_step", optimizer, optimizer_idx)
        trainer._call_lightning_module_hook("on_before_optimizer_step", optimizer, optimizer_idx)
        # TODO: this is done for the entire model but should be changed to per-optimizer
        if optimizer_idx == 0:
            self._track_grad_norm(trainer)
        self._clip_gradients(
            model,
            optimizer,
            optimizer_idx,
            trainer.gradient_clip_val,
            gradient_clip_algorithm=trainer.gradient_clip_algorithm,
        )

    def _wrap_closure(
        self,
        model: "pl.LightningModule",
        optimizer: Optimizer,
        optimizer_idx: int,
        closure: Callable[[], Any],
    ) -> Any:
        """This double-closure allows makes sure the ``closure`` is executed before the
        ``on_before_optimizer_step`` hook is called.

        The closure (generally) runs ``backward`` so this allows inspecting gradients in this hook. This structure is
        consistent with the ``PrecisionPlugin`` subclasses that cannot pass ``optimizer.step(closure)`` directly.
        """
        closure_result = closure()
        self._after_closure(model, optimizer, optimizer_idx)
        return closure_result

    def optimizer_step(
        self,
        model: Union["pl.LightningModule", Module],
        optimizer: Optimizer,
        optimizer_idx: int,
        closure: Callable[[], Any],
        **kwargs: Any,
    ) -> None:
        """Hook to run the optimizer step."""
        if isinstance(model, pl.LightningModule):
            closure = partial(self._wrap_closure, model, optimizer, optimizer_idx, closure)
        optimizer.step(closure=closure, **kwargs)

    def _track_grad_norm(self, trainer: "pl.Trainer") -> None:
        if trainer.track_grad_norm == -1:
            return
        kwargs = {"group_separator": trainer.logger.group_separator} if trainer.logger is not None else {}
        grad_norm_dict = grad_norm(trainer.lightning_module, trainer.track_grad_norm, **kwargs)
        if grad_norm_dict:
            prev_fx = trainer.lightning_module._current_fx_name
            trainer.lightning_module._current_fx_name = "on_before_optimizer_step"
            trainer.lightning_module.log_grad_norm(grad_norm_dict)
            trainer.lightning_module._current_fx_name = prev_fx

    def _clip_gradients(
        self,
        model: Union["pl.LightningModule", Module],
        optimizer: Optimizer,
        optimizer_idx: int,
        clip_val: Optional[Union[int, float]] = None,
        gradient_clip_algorithm: Optional[GradClipAlgorithmType] = None,
    ) -> None:
        if not isinstance(model, pl.LightningModule) or not model.automatic_optimization:
            # the configuration validator disallows clipping on manual
            return
        model.configure_gradient_clipping(
            optimizer,
            optimizer_idx,
            gradient_clip_val=clip_val,
            gradient_clip_algorithm=gradient_clip_algorithm,
        )

    def clip_gradients(
        self,
        optimizer: Optimizer,
        clip_val: Union[int, float] = 0.0,
        gradient_clip_algorithm: GradClipAlgorithmType = GradClipAlgorithmType.NORM,
    ) -> None:
        """Clips the gradients."""
        if clip_val <= 0:
            return
        if gradient_clip_algorithm == GradClipAlgorithmType.VALUE:
            self.clip_grad_by_value(optimizer, clip_val)
        elif gradient_clip_algorithm == GradClipAlgorithmType.NORM:
            self.clip_grad_by_norm(optimizer, clip_val)

    def clip_grad_by_value(self, optimizer: Optimizer, clip_val: Union[int, float]) -> None:
        """Clip gradients by value."""
        parameters = self.main_params(optimizer)
        torch.nn.utils.clip_grad_value_(parameters, clip_value=clip_val)

    def clip_grad_by_norm(self, optimizer: Optimizer, clip_val: Union[int, float]) -> None:
        """Clip gradients by norm."""
        parameters = self.main_params(optimizer)
        torch.nn.utils.clip_grad_norm_(parameters, clip_val)

    def dispatch(self, trainer: "pl.Trainer") -> None:
        """Hook to do something when ``Strategy.dispatch()`` gets called."""

    @contextlib.contextmanager
    def forward_context(self) -> Generator[None, None, None]:
        """A contextmanager for managing model forward/training_step/evaluation_step/predict_step."""
        yield

    @contextlib.contextmanager
    def train_step_context(self) -> Generator[None, None, None]:
        """A contextmanager for the training step."""
        with self.forward_context():
            yield

    @contextlib.contextmanager
    def val_step_context(self) -> Generator[None, None, None]:
        """A contextmanager for the validation step."""
        with self.forward_context():
            yield

    @contextlib.contextmanager
    def test_step_context(self) -> Generator[None, None, None]:
        """A contextmanager for the test step."""
        with self.forward_context():
            yield

    @contextlib.contextmanager
    def predict_step_context(self) -> Generator[None, None, None]:
        """A contextmanager for the predict step."""
        with self.forward_context():
            yield

    def teardown(self) -> None:
        """This method is called to teardown the training process.

        It is the right place to release memory and free other resources.
        """