added a stateful dictionary (#983)

Summary:
Pull Request resolved: #983

Added a stateful dictionary object that implements the stateful interface for checkpoint saving and loading and also extends dict.

This object can be used to store key/value pairs of arbitrary value objects (for example, computed metrics or any other object that we require to be saved to the unit state)

Reviewed By: diego-urgell

Differential Revision: D71051443

fbshipit-source-id: 7a94ae43641a0bc3d93690f015bfa75ca279362b

Author: Rabia Shakoor

torchtnt/utils/stateful.py

@@ -75,3 +75,14 @@ def compute(self) -> Any: ...
     def state_dict(self) -> Dict[str, Any]: ...
 
     def load_state_dict(self, state_dict: Dict[str, Any]) -> None: ...
+
+
+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
+
+    def load_state_dict(self, state_dict: Dict[str, Any]) -> None:
+        self.clear()
+        self.update(state_dict)