feat: oversample sparse VQA annotations (recipe consumption + weighted sampler)

VQA annotations are sparse, so VQA was badly underrepresented in training:
its effective share was weight x density, and blend draws that picked an
ask_vqa* sub-recipe for a non-VQA frame were wasted entirely.

Two pieces:

1. Recipe-side consumption (language_render.py): render_sample now routes
   any frame that carries a VQA annotation to a matching ask_vqa* sub-recipe,
   regardless of the weighted blend draw. No VQA annotation is wasted and no
   draw lands on a non-renderable VQA recipe — VQA's recipe-side share now
   equals the VQA-annotation density.

2. Dataset-side oversampling (WeightedEpisodeAwareSampler + vqa_target_fraction):
   a new weighted, episode-aware sampler draws frames with replacement by
   per-frame weight. When TrainPipelineConfig.vqa_target_fraction is set, the
   train script scans language_events, weights VQA frames so they make up
   ~that fraction of the training stream, and uses the weighted sampler. This
   is what actually lets VQA exceed its natural density. Default None keeps
   uniform episode-aware sampling unchanged.

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

tests/datasets/test_language_render.py

@@ -207,12 +207,8 @@ def test_per_camera_blend_renders_both_views():
             "top": TrainingRecipe(
                 weight=1.0,
                 bindings={
-                    "vqa_query": (
-                        "emitted_at(t, style=vqa, role=user, camera=observation.images.top)"
-                    ),
-                    "vqa": (
-                        "emitted_at(t, style=vqa, role=assistant, camera=observation.images.top)"
-                    ),
+                    "vqa_query": ("emitted_at(t, style=vqa, role=user, camera=observation.images.top)"),
+                    "vqa": ("emitted_at(t, style=vqa, role=assistant, camera=observation.images.top)"),
                 },
                 messages=[
                     MessageTurn(
@@ -236,12 +232,8 @@ def test_per_camera_blend_renders_both_views():
             "wrist": TrainingRecipe(
                 weight=1.0,
                 bindings={
-                    "vqa_query": (
-                        "emitted_at(t, style=vqa, role=user, camera=observation.images.wrist)"
-                    ),
-                    "vqa": (
-                        "emitted_at(t, style=vqa, role=assistant, camera=observation.images.wrist)"
-                    ),
+                    "vqa_query": ("emitted_at(t, style=vqa, role=user, camera=observation.images.wrist)"),
+                    "vqa": ("emitted_at(t, style=vqa, role=assistant, camera=observation.images.wrist)"),
                 },
                 messages=[
                     MessageTurn(
@@ -319,11 +311,19 @@ def test_resolve_task_picks_rephrasing_deterministically_per_sample():
     assert seen == {r["content"] for r in rephrasings}
     # Same sample_idx → same pick (determinism).
     a = render_sample(
-        recipe=recipe, persistent=rephrasings, events=[], t=0.0, sample_idx=42,
+        recipe=recipe,
+        persistent=rephrasings,
+        events=[],
+        t=0.0,
+        sample_idx=42,
         dataset_ctx={"task": "canonical"},
     )
     b = render_sample(
-        recipe=recipe, persistent=rephrasings, events=[], t=0.0, sample_idx=42,
+        recipe=recipe,
+        persistent=rephrasings,
+        events=[],
+        t=0.0,
+        sample_idx=42,
         dataset_ctx={"task": "canonical"},
     )
     assert a["messages"][0]["content"] == b["messages"][0]["content"]
@@ -402,6 +402,52 @@ def test_flow_only_low_level_recipe_renders_without_target():
     assert rendered["target_message_indices"] == []
 
 
+def test_vqa_frame_is_consumed_over_the_weighted_blend():
+    """A frame carrying a VQA annotation renders the ``ask_vqa*`` sub-recipe
+    even when its blend weight is tiny — VQA annotations are sparse and must
+    never be wasted on a subtask/action draw."""
+    recipe = TrainingRecipe(
+        blend={
+            "high_level_subtask": TrainingRecipe(
+                weight=0.99,
+                messages=[
+                    MessageTurn(role="user", content="${task}", stream="high_level"),
+                    MessageTurn(role="assistant", content="a subtask", stream="high_level", target=True),
+                ],
+            ),
+            "ask_vqa_top": TrainingRecipe(
+                weight=0.01,
+                bindings={
+                    "vqa_query": "emitted_at(t, style=vqa, role=user, camera=observation.images.top)",
+                    "vqa": "emitted_at(t, style=vqa, role=assistant, camera=observation.images.top)",
+                },
+                messages=[
+                    MessageTurn(
+                        role="user", content="${vqa_query}", stream="high_level", if_present="vqa_query"
+                    ),
+                    MessageTurn(
+                        role="assistant",
+                        content="${vqa}",
+                        stream="high_level",
+                        target=True,
+                        if_present="vqa",
+                    ),
+                ],
+            ),
+        }
+    )
+    # A frame WITH a vqa event renders VQA on every sample_idx, despite the
+    # ask_vqa weight being only 0.01.
+    for sample_idx in range(20):
+        rendered = render_sample(
+            recipe=recipe, persistent=PERSISTENT, events=EVENTS_AT_1, t=1.0, sample_idx=sample_idx, task="x"
+        )
+        assert rendered["messages"][-1]["content"] == '{"count": 2}', sample_idx
+    # A frame WITHOUT a vqa event falls back to the normal weighted blend.
+    rendered = render_sample(recipe=recipe, persistent=PERSISTENT, events=[], t=1.0, sample_idx=0, task="x")
+    assert rendered["messages"][-1]["content"] == "a subtask"
+
+
 def test_canonical_recipe_can_render_low_level_branch():
     recipe = TrainingRecipe.from_yaml(Path("src/lerobot/configs/recipes/pi05_hirobot.yaml"))
     low_level = TrainingRecipe(blend={"low": recipe.blend["low_level_execution"]})