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

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 OBS_IMAGE, OBS_IMAGES, OBS_STATE
+from lerobot.utils.constants import ACTION, OBS_IMAGE, OBS_IMAGES, OBS_STATE
 from tests.conftest import assert_contract_is_typed
 
 
@@ -257,7 +257,7 @@ def test_step_through_with_dict():
 
     batch = {
         OBS_IMAGE: None,
-        "action": None,
+        ACTION: None,
         "next.reward": 0.0,
         "next.done": False,
         "next.truncated": False,
@@ -1842,7 +1842,7 @@ def test_save_load_with_custom_converter_functions():
         # Verify it uses default converters by checking with standard batch format
         batch = {
             OBS_IMAGE: torch.randn(1, 3, 32, 32),
-            "action": torch.randn(1, 7),
+            ACTION: torch.randn(1, 7),
             "next.reward": torch.tensor([1.0]),
             "next.done": torch.tensor([False]),
             "next.truncated": torch.tensor([False]),
@@ -2094,11 +2094,11 @@ def test_aggregate_joint_action_only():
         patterns=["action.j1.pos", "action.j2.pos"],
     )
 
-    # Expect only "action" with joint names
-    assert "action" in out and OBS_STATE not in out
-    assert out["action"]["dtype"] == "float32"
-    assert set(out["action"]["names"]) == {"j1.pos", "j2.pos"}
-    assert out["action"]["shape"] == (len(out["action"]["names"]),)
+    # Expect only ACTION with joint names
+    assert ACTION in out and OBS_STATE not in out
+    assert out[ACTION]["dtype"] == "float32"
+    assert set(out[ACTION]["names"]) == {"j1.pos", "j2.pos"}
+    assert out[ACTION]["shape"] == (len(out[ACTION]["names"]),)
 
 
 def test_aggregate_ee_action_and_observation_with_videos():
@@ -2113,9 +2113,9 @@ def test_aggregate_ee_action_and_observation_with_videos():
     )
 
     # Action should pack only EE names
-    assert "action" in out
-    assert set(out["action"]["names"]) == {"ee.x", "ee.y"}
-    assert out["action"]["dtype"] == "float32"
+    assert ACTION in out
+    assert set(out[ACTION]["names"]) == {"ee.x", "ee.y"}
+    assert out[ACTION]["dtype"] == "float32"
 
     # Observation state should pack both ee.x and j1.pos as a vector
     assert OBS_STATE in out
@@ -2140,10 +2140,10 @@ def test_aggregate_both_action_types():
         patterns=["action.ee", "action.j1", "action.j2.pos"],
     )
 
-    assert "action" in out
+    assert ACTION in out
     expected = {"ee.x", "ee.y", "j1.pos", "j2.pos"}
-    assert set(out["action"]["names"]) == expected
-    assert out["action"]["shape"] == (len(expected),)
+    assert set(out[ACTION]["names"]) == expected
+    assert out[ACTION]["shape"] == (len(expected),)
 
 
 def test_aggregate_images_when_use_videos_false():