refactor(pipeline): Transition from tuple to dictionary format for EnvTransition

- Updated the EnvTransition structure to use a dictionary format instead of a tuple, enhancing readability and maintainability.
- Replaced instances of TransitionIndex with TransitionKey for accessing transition components.
- Adjusted related processing functions and tests to accommodate the new dictionary format, ensuring consistent handling of transitions across the codebase.

src/lerobot/processor/rename_processor.py

@@ -18,7 +18,7 @@ from typing import Any
 
 import torch
 
-from lerobot.processor.pipeline import EnvTransition, ProcessorStepRegistry, TransitionIndex
+from lerobot.processor.pipeline import EnvTransition, ProcessorStepRegistry, TransitionKey
 
 
 @dataclass
@@ -29,7 +29,7 @@ class RenameProcessor:
     rename_map: dict[str, str] = field(default_factory=dict)
 
     def __call__(self, transition: EnvTransition) -> EnvTransition:
-        observation = transition[TransitionIndex.OBSERVATION]
+        observation = transition.get(TransitionKey.OBSERVATION)
         if observation is None:
             return transition
 
@@ -39,15 +39,11 @@ class RenameProcessor:
                 processed_obs[self.rename_map[key]] = value
             else:
                 processed_obs[key] = value
-        return (
-            processed_obs,
-            transition[TransitionIndex.ACTION],
-            transition[TransitionIndex.REWARD],
-            transition[TransitionIndex.DONE],
-            transition[TransitionIndex.TRUNCATED],
-            transition[TransitionIndex.INFO],
-            transition[TransitionIndex.COMPLEMENTARY_DATA],
-        )
+
+        # Create a new transition with the renamed observation
+        new_transition = transition.copy()
+        new_transition[TransitionKey.OBSERVATION] = processed_obs
+        return new_transition
 
     def get_config(self) -> dict[str, Any]:
         return {"rename_map": self.rename_map}