fix(smolvla2): pass flat batch dict to preprocessor (no manual wrap)

``PolicyProcessorPipeline.__call__`` already wraps its input via
``to_transition`` (defaulting to ``batch_to_transition``) before
running the steps, and unwraps via ``to_output`` (defaulting to
``transition_to_batch``) afterwards. The input format is therefore a
*flat batch dict* keyed by ``observation.*`` / ``action`` / etc., not
an ``EnvTransition``.

Previous attempt pre-wrapped the observation into a transition with
``TransitionKey.OBSERVATION`` as the key, then handed *that* to the
pipeline — which fed it to ``batch_to_transition``, which looked for
top-level ``observation.*`` entries, found none (they were nested
inside the enum key), and produced an empty observation. Every step
then bailed with ``ObservationProcessorStep requires an observation
in the transition.``

Pass the flat dict from ``build_inference_frame`` straight to the
preprocessor — it does the wrap/unwrap itself.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lerobot/scripts/lerobot_smolvla2_runtime.py

@@ -588,7 +588,6 @@ def _build_robot_observation_provider(
         build_inference_frame,
         prepare_observation_for_inference,
     )
-    from lerobot.types import TransitionKey  # noqa: PLC0415
 
     torch_device = torch.device(device) if isinstance(device, str) else device
     robot_type = getattr(robot, "robot_type", None) or getattr(
@@ -629,28 +628,22 @@ def _build_robot_observation_provider(
             return None
 
         if preprocessor is not None:
-            # ``EnvTransition``'s TypedDict is declared with
-            # ``TransitionKey.OBSERVATION.value`` as keys, but every
-            # ProcessorStep in the pipeline does
-            # ``transition.get(TransitionKey.OBSERVATION)`` / indexes
-            # with the *enum member* — not the string ``.value``. Build
-            # the dict with enum keys so the steps actually find the
-            # observation.
-            transition: dict[Any, Any] = {
-                TransitionKey.OBSERVATION: obs_tensors,
-                TransitionKey.ACTION: None,
-                TransitionKey.REWARD: None,
-                TransitionKey.DONE: None,
-                TransitionKey.TRUNCATED: None,
-                TransitionKey.INFO: None,
-                TransitionKey.COMPLEMENTARY_DATA: {},
-            }
+            # ``PolicyProcessorPipeline`` defaults its ``to_transition``
+            # to ``batch_to_transition``, which expects a *flat batch
+            # dict* keyed by ``observation.*`` / ``action`` / etc., and
+            # wraps it into an ``EnvTransition`` itself. Pre-wrapping
+            # here would just have ``batch_to_transition`` look for
+            # ``observation.*`` keys at top level, find none (they'd
+            # be nested under ``TransitionKey.OBSERVATION``), and
+            # produce an empty observation → ``ObservationProcessorStep``
+            # bails. Pass the flat dict straight in; ``to_output``
+            # gives us a flat dict back.
             try:
-                transition = preprocessor(transition)
+                processed = preprocessor(obs_tensors)
             except Exception as exc:  # noqa: BLE001
                 logger.warning("preprocessor failed on robot observation: %s", exc)
                 return None
-            obs_tensors = transition.get(TransitionKey.OBSERVATION) or {}
+            obs_tensors = processed if isinstance(processed, dict) else {}
 
         observation = {
             k: v