review: decode keyframes via video_utils.decode_video_frames

Addresses three of CarolinePascal's frames.py comments (the fourth, the
subprocess re-encode, waits on #3611):

- replace the bespoke _decode_pyav_direct PyAV decoder with
  lerobot.datasets.video_utils.decode_video_frames (torchcodec backend,
  PyAV fallback) — torchvision's VideoReader removal no longer applies
- frames flow through the provider as torch.Tensor (C, H, W uint8); PIL
  is materialised only at the VLM-message boundary in to_image_blocks /
  to_video_block, where the chat backends need it
- _decode now returns exactly one frame per timestamp (or [] on failure),
  so frames_at pairs them with strict=True

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

tests/annotations/test_frames.py

@@ -15,13 +15,12 @@
 # limitations under the License.
 """Unit tests for :class:`VideoFrameProvider` method bindings.
 
-These were prompted by a real regression: ``video_for_episode`` was
-indented one level too deep so it ended up nested *inside* the
-``_decode_pyav_direct`` helper (after that function's ``return``
-statement) — silently dead code that meant production runs with
-``use_video_url=False`` would ``AttributeError`` on
-``self.frame_provider.video_for_episode(...)``. The existing module
-tests didn't catch it because they exercise stub providers.
+These were prompted by a real regression: ``video_for_episode`` was once
+indented one level too deep so it ended up nested *inside* a module-level
+helper (after that function's ``return`` statement) — silently dead code
+that meant production runs with ``use_video_url=False`` would
+``AttributeError`` on ``self.frame_provider.video_for_episode(...)``. The
+existing module tests didn't catch it because they exercise stub providers.
 
 The tests below assert on the class itself (not on an instance), so a
 future reindent regression flips them to red without needing a real
@@ -51,8 +50,8 @@ def test_episode_clip_path_is_a_method_of_videoframeprovider():
 
 
 def test_videoframeprovider_has_a_lock_for_concurrent_use():
-    """A ``ThreadPoolExecutor`` runs Module 1/2/3 phases concurrently;
-    the cache + warn-flag accesses must be guarded.
+    """A ``ThreadPoolExecutor`` runs the plan / interjections / vqa phases
+    concurrently; the cache + warn-flag accesses must be guarded.
     """
     import threading