chore: replace hard-coded obs values with constants throughout all the source code (#2037)

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

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

tests/datasets/test_datasets.py

@@ -46,6 +46,7 @@ from lerobot.datasets.utils import (
 from lerobot.envs.factory import make_env_config
 from lerobot.policies.factory import make_policy_config
 from lerobot.robots import make_robot_from_config
+from lerobot.utils.constants import OBS_IMAGES, OBS_STATE, OBS_STR
 from tests.fixtures.constants import DUMMY_CHW, DUMMY_HWC, DUMMY_REPO_ID
 from tests.mocks.mock_robot import MockRobotConfig
 from tests.utils import require_x86_64_kernel
@@ -75,7 +76,7 @@ def test_same_attributes_defined(tmp_path, lerobot_dataset_factory):
     # Instantiate both ways
     robot = make_robot_from_config(MockRobotConfig())
     action_features = hw_to_dataset_features(robot.action_features, "action", True)
-    obs_features = hw_to_dataset_features(robot.observation_features, "observation", True)
+    obs_features = hw_to_dataset_features(robot.observation_features, OBS_STR, True)
     dataset_features = {**action_features, **obs_features}
     root_create = tmp_path / "create"
     dataset_create = LeRobotDataset.create(
@@ -397,7 +398,7 @@ def test_factory(env_name, repo_id, policy_name):
         ("frame_index", 0, True),
         ("timestamp", 0, True),
         # TODO(rcadene): should we rename it agent_pos?
-        ("observation.state", 1, True),
+        (OBS_STATE, 1, True),
         ("next.reward", 0, False),
         ("next.done", 0, False),
     ]
@@ -662,7 +663,7 @@ def test_check_cached_episodes_sufficient(tmp_path, lerobot_dataset_factory):
 def test_update_chunk_settings(tmp_path, empty_lerobot_dataset_factory):
     """Test the update_chunk_settings functionality for both LeRobotDataset and LeRobotDatasetMetadata."""
     features = {
-        "observation.state": {
+        OBS_STATE: {
             "dtype": "float32",
             "shape": (6,),
             "names": ["shoulder_pan", "shoulder_lift", "elbow", "wrist_1", "wrist_2", "wrist_3"],
@@ -769,7 +770,7 @@ def test_update_chunk_settings(tmp_path, empty_lerobot_dataset_factory):
 def test_update_chunk_settings_video_dataset(tmp_path):
     """Test update_chunk_settings with a video dataset to ensure video-specific logic works."""
     features = {
-        "observation.images.cam": {
+        f"{OBS_IMAGES}.cam": {
             "dtype": "video",
             "shape": (480, 640, 3),
             "names": ["height", "width", "channels"],