chore: replace hard-coded next values with constants throughout all the source code (#2056)

tests/utils/test_replay_buffer.py

@@ -22,7 +22,7 @@ import torch
 
 from lerobot.datasets.lerobot_dataset import LeRobotDataset
 from lerobot.rl.buffer import BatchTransition, ReplayBuffer, random_crop_vectorized
-from lerobot.utils.constants import ACTION, OBS_IMAGE, OBS_STATE, OBS_STR
+from lerobot.utils.constants import ACTION, DONE, OBS_IMAGE, OBS_STATE, OBS_STR, REWARD
 from tests.fixtures.constants import DUMMY_REPO_ID
 
 
@@ -380,9 +380,9 @@ def test_to_lerobot_dataset(tmp_path):
         for feature, value in ds[i].items():
             if feature == ACTION:
                 assert torch.equal(value, buffer.actions[i])
-            elif feature == "next.reward":
+            elif feature == REWARD:
                 assert torch.equal(value, buffer.rewards[i])
-            elif feature == "next.done":
+            elif feature == DONE:
                 assert torch.equal(value, buffer.dones[i])
             elif feature == OBS_IMAGE:
                 # Tensor -> numpy is not precise, so we have some diff there