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

tests/processor/test_pipeline.py

@@ -35,7 +35,7 @@ from lerobot.processor import (
     TransitionKey,
 )
 from lerobot.processor.converters import create_transition, identity_transition
-from lerobot.utils.constants import ACTION, OBS_IMAGE, OBS_IMAGES, OBS_STATE
+from lerobot.utils.constants import ACTION, DONE, OBS_IMAGE, OBS_IMAGES, OBS_STATE, REWARD, TRUNCATED
 from tests.conftest import assert_contract_is_typed
 
 
@@ -258,9 +258,9 @@ def test_step_through_with_dict():
     batch = {
         OBS_IMAGE: None,
         ACTION: None,
-        "next.reward": 0.0,
-        "next.done": False,
-        "next.truncated": False,
+        REWARD: 0.0,
+        DONE: False,
+        TRUNCATED: False,
         "info": {},
     }
 
@@ -1843,9 +1843,9 @@ def test_save_load_with_custom_converter_functions():
         batch = {
             OBS_IMAGE: torch.randn(1, 3, 32, 32),
             ACTION: torch.randn(1, 7),
-            "next.reward": torch.tensor([1.0]),
-            "next.done": torch.tensor([False]),
-            "next.truncated": torch.tensor([False]),
+            REWARD: torch.tensor([1.0]),
+            DONE: torch.tensor([False]),
+            TRUNCATED: torch.tensor([False]),
             "info": {},
         }