chore: fixing type hint checkpointing class (#590)

This PR fixes the type hint of some classes in the checkpointing code.

torchtitan/checkpoint.py

@@ -84,7 +84,7 @@ class ModelWrapper(Stateful):
     def __init__(self, model: Union[nn.Module, List[nn.Module]]) -> None:
         self.model = [model] if isinstance(model, nn.Module) else model
 
-    def state_dict(self) -> None:
+    def state_dict(self) -> Dict[str, Any]:
         return {
             k: v for sd in map(get_model_state_dict, self.model) for k, v in sd.items()
         }
@@ -107,7 +107,7 @@ def __init__(
         self.model = [model] if isinstance(model, nn.Module) else model
         self.optim = [optim] if isinstance(optim, torch.optim.Optimizer) else optim
 
-    def state_dict(self) -> None:
+    def state_dict(self) -> Dict[str, Any]:
         func = functools.partial(
             get_optimizer_state_dict,
             options=StateDictOptions(flatten_optimizer_state_dict=True),