Added getstate/setstate method for torch.save serialization (#4127)

* Added getstate/setstate method for torch.save serialization, added additional Optional Typing to results object

* Added tests to ensure torch.save does not fail

* Added flags to ensure compatible ddp cpu environment

* Removed torch version check due to minimum already being 1.3, reduced epochs for speed

* Moved tests to separate file

* Update to accelerator, move to ddp_spawn to prevent hanging ddp

pytorch_lightning/accelerators/accelerator.py

@@ -214,6 +214,20 @@ def init_ddp_connection(
                 torch_backend, rank=global_rank, world_size=world_size
             )
 
+    def __getstate__(self):
+        return {
+            'trainer': self.trainer,
+            'nickname': self.nickname,
+            'cluster_environment': self.cluster_environment,
+            'dist': self.dist
+        }
+
+    def __setstate__(self, d):
+        self.trainer = d['trainer']
+        self.nickname = d['nickname']
+        self.cluster_environment = d['cluster_environment']
+        self.dist = d['dist']
+
 
 # TODO: allow user to compare with string even internaly we shall use these Enum to prevent typos...
 class BackendType(Enum):

pytorch_lightning/core/lightning.py

@@ -107,7 +107,7 @@ def __init__(self, *args, **kwargs):
         # optionally can be set by user
         self._example_input_array = None
         self._datamodule = None
-        self._results: Result = None
+        self._results: Optional[Result] = None
         self._current_fx_name = ''
 
     def optimizers(self):

pytorch_lightning/core/step_result.py

@@ -23,6 +23,7 @@
 from pytorch_lightning.utilities.distributed import sync_ddp_if_available
 from pytorch_lightning.metrics import Metric
 
+
 class Result(Dict):
     def __init__(
         self,
@@ -89,6 +90,12 @@ def __setattr__(self, key: str, val: Union[Tensor, Any]):
 
         self[key] = val
 
+    def __getstate__(self):
+        return self
+
+    def __setstate__(self, d):
+        self.update(d)
+
     def _assert_tensor_metric(self, name: str, potential_metric: Union[bool, Tensor, None, Any]):
         if potential_metric is not None and not isinstance(potential_metric, bool):
             assert isinstance(potential_metric, Tensor), f'{name} must be a torch.Tensor'

tests/checkpointing/test_model_checkpoint.py

@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import os
+from distutils.version import LooseVersion
 from unittest.mock import MagicMock, Mock
 
 import yaml

tests/checkpointing/test_torch_saving.py

@@ -0,0 +1,76 @@
+# 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 os
+import platform
+
+import pytest
+import torch
+
+from pytorch_lightning import Trainer
+from tests.base import EvalModelTemplate
+
+
+def test_model_torch_save(tmpdir):
+    """Test to ensure torch save does not fail for model and trainer."""
+    model = EvalModelTemplate()
+    num_epochs = 1
+    trainer = Trainer(
+        default_root_dir=tmpdir,
+        max_epochs=num_epochs,
+    )
+    temp_path = os.path.join(tmpdir, 'temp.pt')
+    trainer.fit(model)
+
+    # Ensure these do not fail
+    torch.save(trainer.model, temp_path)
+    torch.save(trainer, temp_path)
+
+
+@pytest.mark.skipif(platform.system() == "Windows",
+                    reason="Distributed training is not supported on Windows")
+def test_model_torch_save_ddp_cpu(tmpdir):
+    """Test to ensure torch save does not fail for model and trainer using cpu ddp."""
+    model = EvalModelTemplate()
+    num_epochs = 1
+    trainer = Trainer(
+        default_root_dir=tmpdir,
+        max_epochs=num_epochs,
+        accelerator="ddp_cpu",
+        num_processes=2,
+    )
+    temp_path = os.path.join(tmpdir, 'temp.pt')
+    trainer.fit(model)
+
+    # Ensure these do not fail
+    torch.save(trainer.model, temp_path)
+    torch.save(trainer, temp_path)
+
+
+@pytest.mark.skipif(torch.cuda.device_count() < 2, reason="test requires multi-GPU machine")
+def test_model_torch_save_ddp_cuda(tmpdir):
+    """Test to ensure torch save does not fail for model and trainer using gpu ddp."""
+    model = EvalModelTemplate()
+    num_epochs = 1
+    trainer = Trainer(
+        default_root_dir=tmpdir,
+        max_epochs=num_epochs,
+        accelerator="ddp_spawn",
+        gpus=2
+    )
+    temp_path = os.path.join(tmpdir, 'temp.pt')
+    trainer.fit(model)
+
+    # Ensure these do not fail
+    torch.save(trainer.model, temp_path)
+    torch.save(trainer, temp_path)