Feat/pipeline add feature contract (#1637)

* Add feature contract to pipelinestep and pipeline

* Add tests

* Add processor tests

* PR feedback

* encorperate pr feedback

* type in doc

* oops

src/lerobot/processor/device_processor.py

@@ -18,6 +18,7 @@ from typing import Any
 
 import torch
 
+from lerobot.configs.types import PolicyFeature
 from lerobot.processor.pipeline import EnvTransition, TransitionKey
 
 
@@ -74,3 +75,6 @@ class DeviceProcessor:
     def get_config(self) -> dict[str, Any]:
         """Return configuration for serialization."""
         return {"device": self.device}
+
+    def feature_contract(self, features: dict[str, PolicyFeature]) -> dict[str, PolicyFeature]:
+        return features