chore: replace hard-coded action values with constants throughout all the source code (#2055)

* chore: replace hard-coded 'action' values with constants throughout all the source code

* chore(tests): replace hard-coded action values with constants throughout all the test code

src/lerobot/processor/converters.py

@@ -23,7 +23,7 @@ from typing import Any
 import numpy as np
 import torch
 
-from lerobot.utils.constants import OBS_PREFIX
+from lerobot.utils.constants import ACTION, OBS_PREFIX
 
 from .core import EnvTransition, PolicyAction, RobotAction, RobotObservation, TransitionKey
 
@@ -344,7 +344,7 @@ def batch_to_transition(batch: dict[str, Any]) -> EnvTransition:
     if not isinstance(batch, dict):
         raise ValueError(f"EnvTransition must be a dictionary. Got {type(batch).__name__}")
 
-    action = batch.get("action")
+    action = batch.get(ACTION)
     if action is not None and not isinstance(action, PolicyAction):
         raise ValueError(f"Action should be a PolicyAction type got {type(action)}")
 
@@ -354,7 +354,7 @@ def batch_to_transition(batch: dict[str, Any]) -> EnvTransition:
 
     return create_transition(
         observation=observation_keys if observation_keys else None,
-        action=batch.get("action"),
+        action=batch.get(ACTION),
         reward=batch.get("next.reward", 0.0),
         done=batch.get("next.done", False),
         truncated=batch.get("next.truncated", False),
@@ -379,7 +379,7 @@ def transition_to_batch(transition: EnvTransition) -> dict[str, Any]:
         raise ValueError(f"Transition should be a EnvTransition type (dict) got {type(transition)}")
 
     batch = {
-        "action": transition.get(TransitionKey.ACTION),
+        ACTION: transition.get(TransitionKey.ACTION),
         "next.reward": transition.get(TransitionKey.REWARD, 0.0),
         "next.done": transition.get(TransitionKey.DONE, False),
         "next.truncated": transition.get(TransitionKey.TRUNCATED, False),