Bug fix: DictStateful fails to load state_dict (#1028)

Summary:
Pull Request resolved: #1028

### Bug Fix: DictStateful Fails to Load State Dict

#### Overview

This diff addresses a bug in the `DictStateful` class where it fails to load the state dictionary. The fix involves modifying the `state_dict` method to return a copy of the dictionary instead of the original dictionary. Additionally, the `load_state_dict` method is updated to clear the dictionary before loading the new state.

#### Changes

*   `fbcode/torchtnt/utils/stateful.py`:
    *   The `state_dict` method is updated to return a copy of the dictionary (`self.copy()`) instead of the original dictionary (`self`).
*   `fbcode/torchtnt/tests/utils/meta/test_stateful.py`:
    *   Test cases are added to verify the correctness of the `DictStateful` class, including a test for checkpointing.

#### Impact

This bug fix ensures that the `DictStateful` class functions correctly when saving and loading its state dictionary. This is crucial for maintaining the accuracy and reliability of machine learning models that rely on this class.

Reviewed By: JKSenthil

Differential Revision: D81674115

fbshipit-source-id: 6ee8e8a5d91733dd46a59198316554cff68f8b7f

Author: IshanSengarUnify

torchtnt/utils/stateful.py

@@ -79,7 +79,7 @@ class DictStateful(Stateful, Dict[str, Any]):
     """A dictionary that implements the stateful interface that can be saved and loaded from checkpoints."""
 
     def state_dict(self) -> Dict[str, Any]:
-        return self
+        return self.copy()
 
     def load_state_dict(self, state_dict: Dict[str, Any]) -> None:
         self.clear()