feat(batch_processor): Add feature_contract method to ToBatchProcessor

- Introduced feature_contract method that returns features without modification, maintaining the no-op behavior of the processor.
- This addition enhances the flexibility of the ToBatchProcessor for future feature processing needs.

src/lerobot/processor/batch_processor.py

@@ -121,3 +121,7 @@ class ToBatchProcessor:
     def reset(self) -> None:
         """Reset processor state (no-op for this processor)."""
         pass
+
+    def feature_contract(self, features: dict[str, Any]) -> dict[str, Any]:
+        """Return features (no-op for this processor)."""
+        return features