update model interface to have __call__ method

Author: curtischong

torch_sim/models/interface.py

@@ -169,6 +169,10 @@ def forward(self, state: SimState | StateDict, **kwargs) -> dict[str, torch.Tens
             ```
         """
 
+    @abstractmethod
+    def __call__(*args, **kwargs) -> dict[str, torch.Tensor]:
+        """Where the input is fed into the model. This is to help typecheckers."""
+
 
 def validate_model_outputs(  # noqa: C901, PLR0915
     model: ModelInterface, device: torch.device, dtype: torch.dtype