Only make a shallow copy when loading optimizer state_dict (#106082)

The thing we do still deep copy is the param_groups, which is much lighter weight. This should also save memory when loading from a checkpoint.

The deepcopy was introduced in ecfcf39f30, but module.py had only a shallow copy at that point so it did not actually bring parity.

Incorporates an XLA fix, which is why I'm updating the pin to ca5eab87a7

Pull Request resolved: https://github.com/pytorch/pytorch/pull/106082
Approved by: https://github.com/albanD, https://github.com/Skylion007

.git-blame-ignore-revs

@@ -36,3 +36,5 @@ dd3a77bc965adf9fe8ba582ee13bb7f14c9661b0
 f70844bec783bfce43c950ccf180dc494e86f2bf
 # 2023-07-28 Apply UFMT to all non test/torch files
 e6ec0efaf87703c5f889cfc20b29be455885d58d
+# 2023-07-31 [optim][BE] split test file into logical parts: SWA, LR, optim
+a53cda1ddc15336dc1ff0ce1eff2a49cdc5f882e

.github/ci_commit_pins/xla.txt

@@ -1 +1 @@
-f5edcb2088195db71bcd36d0f8f1b6a5e663afd8
+ca5eab87a71f80cd3168630511d02549cc7d2516

test/optim/test_optim.py

@@ -238,8 +238,6 @@ class TestOptim(TestCase):
             optimizer_c.step(fn_c)
             self.assertEqual(weight, weight_c)
             self.assertEqual(bias, bias_c)
-        # Make sure state dict wasn't modified
-        self.assertEqual(state_dict, state_dict_c)
         # Make sure state dict is deterministic with equal but not identical parameters
         self.assertEqual(optimizer.state_dict(), optimizer_c.state_dict())
         # Make sure repeated parameters have identical representation in state dict
@@ -301,7 +299,7 @@ class TestOptim(TestCase):
         state_dict_c = deepcopy(optimizer.state_dict())
         optimizer_cuda.load_state_dict(state_dict_c)
 
-        # Make sure state dict wasn't modified
+        # Make sure state_dict_c isn't modified by merely calling load_state_dict
         self.assertEqual(state_dict, state_dict_c)
 
         # Make sure that device of state['step'] is still CPU
@@ -312,7 +310,7 @@ class TestOptim(TestCase):
             for state in new_state_dict["state"].values():
                 self.assertEqual(state["step"].device.type, "cpu")
 
-        for _i in range(20):
+        for _ in range(20):
             optimizer.step(fn)
             optimizer_cuda.step(fn_cuda)
             self.assertEqual(weight, weight_cuda)

torch/optim/optimizer.py

@@ -712,8 +712,8 @@ class Optimizer:
             state_dict (dict): optimizer state. Should be an object returned
                 from a call to :meth:`state_dict`.
         """
-        # deepcopy, to be consistent with module API
-        state_dict = deepcopy(state_dict)
+        # shallow copy, to be consistent with module API
+        state_dict = state_dict.copy()
 
         for pre_hook in self._optimizer_load_state_dict_pre_hooks.values():
             hook_result = pre_hook(self, state_dict)
@@ -722,7 +722,9 @@ class Optimizer:
 
         # Validate the state_dict
         groups = self.param_groups
-        saved_groups = state_dict['param_groups']
+
+        # Deepcopy as we write into saved_groups later to update state
+        saved_groups = deepcopy(state_dict['param_groups'])
 
         if len(groups) != len(saved_groups):
             raise ValueError("loaded state dict has a different number of "