fix(smolvla2): enqueue full chunk via predict_action_chunk

``LowLevelForward`` was calling ``select_action()`` once per
``chunk_hz`` tick. SmolVLA's ``select_action`` is a thin queue-pop:
it returns one action per call and only re-runs the expensive
flow-matching forward when its private internal queue empties.
Result: we got one action back per chunk_hz tick (1Hz default),
``DispatchAction`` at ctrl_hz=30 popped it instantly, then queue
sat empty for ~1s waiting for the next tick. Net throughput was
1 dispatched action/sec instead of the 30 we wanted.

Switch to ``predict_action_chunk`` and enqueue every step of the
returned ``(batch, n_action_steps, action_dim)`` chunk. Refresh
only when the queue is below half a chunk so we don't burn one
flow-matching forward per chunk_hz tick — saves ~5x inference cost
on this hot path. At ctrl_hz=30, chunk_size=50, the queue drains
in ~1.7s before the next refresh, giving smooth dispatch at the
control rate the robot was trained on.

Side effect: ``state['last_chunk_size']`` records how many actions
the most recent chunk produced — useful for the panel later if we
want to surface "chunks generated" alongside "dispatched".

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

src/lerobot/policies/smolvla2/inference/steps.py

@@ -92,17 +92,28 @@ class LowLevelForward(InferenceStep):
         if self.policy is None or self.observation_provider is None:
             return None
         if not state.get("task"):
-            # No task yet → nothing useful to condition on.
             return None
+
+        # SmolVLA produces *action chunks* (typically 50 steps via
+        # flow-matching). The expensive part is the chunk forward;
+        # popping one action per dispatch tick is essentially free.
+        # Only regenerate when the queue is low so we don't burn one
+        # full chunk forward per chunk_hz tick when most of the
+        # previous chunk is still buffered.
+        queue = state.setdefault("action_queue", [])
+        chunk_size = getattr(self.policy.config, "chunk_size", None) or getattr(
+            self.policy.config, "n_action_steps", 50
+        )
+        # Refresh threshold: keep at least half a chunk buffered.
+        if len(queue) > max(1, chunk_size // 2):
+            return None
+
         observation = self.observation_provider()
         if observation is None:
             return None
-        # SmolVLA's ``select_action`` expects the full preprocessed
-        # batch, including ``OBS_LANGUAGE_TOKENS`` /
-        # ``OBS_LANGUAGE_ATTENTION_MASK``. The observation provider
-        # only returns image / state features (the runtime drives
-        # messages itself), so build a low-level prompt from current
-        # runtime state and tokenize it inline.
+
+        # Same prompt construction as before — task + plan + memory,
+        # optional current subtask — then merge into the obs batch.
         ctx = _control_context_messages(state)
         if state.get("current_subtask"):
             ctx = ctx + [{"role": "assistant", "content": state["current_subtask"]}]
@@ -115,16 +126,38 @@ class LowLevelForward(InferenceStep):
         observation = dict(observation)
         observation[OBS_LANGUAGE_TOKENS] = text_batch["lang_tokens"]
         observation[OBS_LANGUAGE_ATTENTION_MASK] = text_batch["lang_masks"]
+
         try:
-            action = self.policy.select_action(observation)
+            # ``predict_action_chunk`` returns the *full* chunk shape
+            # ``(batch, n_action_steps, action_dim)``. Enqueue every
+            # step so DispatchAction at ctrl_hz can drain them
+            # smoothly until the next refresh.
+            chunk = self.policy.predict_action_chunk(observation)
         except Exception as exc:  # noqa: BLE001
-            logger.warning("select_action failed: %s", exc, exc_info=logger.isEnabledFor(logging.DEBUG))
-            push_log(state, f"  [warn] select_action failed: {type(exc).__name__}: {exc}")
+            logger.warning(
+                "predict_action_chunk failed: %s",
+                exc,
+                exc_info=logger.isEnabledFor(logging.DEBUG),
+            )
+            push_log(
+                state,
+                f"  [warn] predict_action_chunk failed: "
+                f"{type(exc).__name__}: {exc}",
+            )
             return None
-        # SmolVLA returns a single action; if the underlying policy
-        # streams chunks, split per-step here. For v1 we just enqueue
-        # the result.
-        state.setdefault("action_queue", []).append(action)
+
+        # ``chunk`` shape: ``(batch, n_action_steps, action_dim)``. Push
+        # each step as a ``(1, action_dim)`` tensor so the existing
+        # action executor's batch-squeeze logic works unchanged.
+        if chunk.ndim == 3:
+            chunk_iter = chunk[0]  # ``(n_action_steps, action_dim)``
+        elif chunk.ndim == 2:
+            chunk_iter = chunk
+        else:
+            chunk_iter = chunk.unsqueeze(0)
+        for step in chunk_iter:
+            queue.append(step.unsqueeze(0))
+        state["last_chunk_size"] = int(chunk_iter.shape[0])
         return None