Consolidate state when retrieving sharded state dict in Lite (#10746)

Co-authored-by: thomas chaton <[email protected]>

CHANGELOG.md

@@ -29,6 +29,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed signals being registered within threads ([#10610](https://github.com/PyTorchLightning/pytorch-lightning/pull/10610))
 - Fixed an issue with collecting logged test results with multiple dataloaders ([#10522](https://github.com/PyTorchLightning/pytorch-lightning/pull/10522))
 
+- Fixed a consolidation error in Lite when attempting to save the state dict of a sharded optimizer ([#10746](https://github.com/PyTorchLightning/pytorch-lightning/pull/10746))
+
+
 - Fixed `_compare_version` for python packages ([#10762](https://github.com/PyTorchLightning/pytorch-lightning/pull/10762))
 
 

pytorch_lightning/lite/wrappers.py

@@ -46,7 +46,7 @@ def __init__(self, optimizer: Optimizer, accelerator: Accelerator) -> None:
         """
         # `__del__` is skipped in case the optimizer has implemented custom destructor logic which we would
         # not want to call on destruction of the `_LiteOptimizer
-        self.__dict__ = {k: v for k, v in optimizer.__dict__.items() if k not in ("step", "__del__")}
+        self.__dict__ = {k: v for k, v in optimizer.__dict__.items() if k not in ("state_dict", "step", "__del__")}
         self.__class__ = type("Lite" + optimizer.__class__.__name__, (self.__class__, optimizer.__class__), {})
         self._optimizer = optimizer
         self._accelerator = accelerator
@@ -55,6 +55,9 @@ def __init__(self, optimizer: Optimizer, accelerator: Accelerator) -> None:
     def optimizer(self) -> Optimizer:
         return self._optimizer
 
+    def state_dict(self) -> Dict[str, Tensor]:
+        return self._accelerator.optimizer_state(self.optimizer)
+
     def step(self, closure: Optional[Callable] = None) -> None:
         closure = closure or _do_nothing_closure
         self._accelerator.optimizer_step(

tests/lite/test_wrappers.py

@@ -142,6 +142,15 @@ def test_lite_optimizer_wraps():
     assert isinstance(lite_optimizer, optimizer_cls)
 
 
+def test_lite_optimizer_state_dict():
+    """Test that the LiteOptimizer calls into the accelerator/strategy to collect the state."""
+    optimizer = Mock()
+    accelerator = Mock()
+    lite_optimizer = _LiteOptimizer(optimizer=optimizer, accelerator=accelerator)
+    lite_optimizer.state_dict()
+    accelerator.optimizer_state.assert_called_with(optimizer)
+
+
 def test_lite_optimizer_steps():
     """Test that the LiteOptimizer forwards the step() and zero_grad() calls to the wrapped optimizer."""
     optimizer = Mock()