Refactor LightningDataParallel (#5670)

* module

* fix model access

* scalar conversion

* refactor

* kwargs

* auto unsqueeze

* refactor code duplication

* clean up

* docs

* update dp docs

* changelog

* generalize test

* test

* rename

* warning cache

* isort

* unsqueezing test

* device

* device

* scalar test

* device

* device

* include coverage of overrides

* clear

* add deprecation test

* docs

* improve coverage

* increase coverage

* fix merge

* extend test

* rename base class

* mention the predict method in docs

* combine iteration over collection

* remove override

* move

* line

* Apply suggestions from code review

* fix running stage

* f401

* fix cyclic import

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Jirka Borovec <Borda@users.noreply.github.com>

CHANGELOG.md

@@ -120,6 +120,12 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Moved accelerators and plugins to its `legacy` pkg ([#5645](https://github.com/PyTorchLightning/pytorch-lightning/pull/5645))
 
 
+- Deprecated `LightningDistributedDataParallel` in favor of new wrapper module `LightningDistributedModule` ([#5185](https://github.com/PyTorchLightning/pytorch-lightning/pull/5185))
+
+
+- Deprecated `LightningDataParallel` in favor of new wrapper module `LightningParallelModule` ([#5670](https://github.com/PyTorchLightning/pytorch-lightning/pull/5670))
+
+
 ### Removed
 
 - Removed deprecated checkpoint argument `filepath` ([#5321](https://github.com/PyTorchLightning/pytorch-lightning/pull/5321))

pytorch_lightning/accelerators/legacy/dp_accelerator.py

@@ -21,7 +21,7 @@
 from pytorch_lightning.core.lightning import LightningModule
 from pytorch_lightning.core.step_result import Result
 from pytorch_lightning.distributed import LightningDistributed
-from pytorch_lightning.overrides.data_parallel import LightningDataParallel
+from pytorch_lightning.overrides.data_parallel import LightningParallelModule
 from pytorch_lightning.utilities import AMPType
 from pytorch_lightning.utilities.exceptions import MisconfigurationException
 
@@ -74,7 +74,7 @@ def __init_torch_data_parallel(self, model):
 
         # set dp device
         torch.cuda.set_device(self.trainer.root_gpu)
-        model = LightningDataParallel(model, device_ids=device_ids)
+        model = torch.nn.DataParallel(LightningParallelModule(model), device_ids=device_ids)
         return model
 
     def __init_half_precision(self, model):
@@ -181,8 +181,10 @@ def reinit_scheduler_properties(self, optimizers: list, schedulers: list):
                     scheduler.load_state_dict(state)
 
     def get_reference_model(self, model) -> LightningModule:
-        if isinstance(model, LightningDataParallel):
-            return model.module
+        if isinstance(model, torch.nn.DataParallel):
+            model = model.module
+        if isinstance(model, LightningParallelModule):
+            model = model.module
         return model
 
     @property

pytorch_lightning/overrides/__init__.py

@@ -0,0 +1,2 @@
+from pytorch_lightning.overrides.data_parallel import LightningParallelModule  # noqa: F401
+from pytorch_lightning.overrides.distributed import LightningDistributedModule  # noqa: F401

pytorch_lightning/overrides/base.py

@@ -0,0 +1,63 @@
+# 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.
+from typing import Any
+
+import torch
+
+from pytorch_lightning.core.lightning import LightningModule
+from pytorch_lightning.trainer.states import RunningStage
+from pytorch_lightning.utilities.warnings import WarningCache
+
+warning_cache = WarningCache()
+
+
+class _LightningModuleWrapperBase(torch.nn.Module):
+
+    def __init__(self, pl_module: LightningModule):
+        """
+        Wraps the user's LightningModule and redirects the forward call to the appropriate
+        method, either ``training_step``, ``validation_step`` or ``test_step``.
+        If the LightningModule is in none of the states `training`, `testing` or `validation`,
+        the inputs will be redirected to the
+        :meth:`~pytorch_lightning.core.lightning.LightningModule.predict` method.
+        Inheriting classes may also modify the inputs or outputs of forward.
+
+        Args:
+            pl_module: the model to wrap
+        """
+        super().__init__()
+        self.module = pl_module
+
+    def forward(self, *inputs, **kwargs):
+        running_stage = self.module.running_stage
+
+        if running_stage == RunningStage.TRAINING:
+            output = self.module.training_step(*inputs, **kwargs)
+            warn_if_output_is_none(output, "training_step")
+        elif running_stage == RunningStage.TESTING:
+            output = self.module.test_step(*inputs, **kwargs)
+            warn_if_output_is_none(output, "test_step")
+        elif running_stage == RunningStage.EVALUATING:
+            output = self.module.validation_step(*inputs, **kwargs)
+            warn_if_output_is_none(output, "validation_step")
+        else:
+            output = self.module.predict(*inputs, **kwargs)
+
+        return output
+
+
+def warn_if_output_is_none(output: Any, method_name: str) -> None:
+    """ Warns user about which method returned None. """
+    if output is None:
+        warning_cache.warn(f'Your {method_name} returned None. Did you forget to return an output?')