fix(video): replace assertions with proper exceptions in video frame decoding (#3016)

Replaced assert statements with FrameTimestampError exceptions in
decode_video_frames_torchvision and decode_video_frames_torchcodec.

Assertions are unsuitable for runtime validation because they can be
silently disabled with python -O, and they produce unhelpful
AssertionError tracebacks. The codebase already defines
FrameTimestampError for this exact purpose but it was only used
in one of the three validation sites.

Also removed AssertionError from the except clause in
LeRobotDataset.__init__, which was masking video timestamp errors
by silently triggering a dataset re-download instead of surfacing
the actual problem.

src/lerobot/datasets/lerobot_dataset.py

@@ -747,7 +747,7 @@ class LeRobotDataset(torch.utils.data.Dataset):
             # Check if cached dataset contains all requested episodes
             if not self._check_cached_episodes_sufficient():
                 raise FileNotFoundError("Cached dataset doesn't contain all requested episodes")
-        except (AssertionError, FileNotFoundError, NotADirectoryError):
+        except (FileNotFoundError, NotADirectoryError):
             if is_valid_version(self.revision):
                 self.revision = get_safe_version(self.repo_id, self.revision)
             self.download(download_videos)

src/lerobot/datasets/video_utils.py

@@ -227,11 +227,12 @@ def decode_video_frames_torchvision(
     min_, argmin_ = dist.min(1)
 
     is_within_tol = min_ < tolerance_s
-    assert is_within_tol.all(), (
+    if not is_within_tol.all():
+        raise FrameTimestampError(
             f"One or several query timestamps unexpectedly violate the tolerance ({min_[~is_within_tol]} > {tolerance_s=})."
-        "It means that the closest frame that can be loaded from the video is too far away in time."
+            " It means that the closest frame that can be loaded from the video is too far away in time."
-        "This might be due to synchronization issues with timestamps during data collection."
+            " This might be due to synchronization issues with timestamps during data collection."
-        "To be safe, we advise to ignore this item during training."
+            " To be safe, we advise to ignore this item during training."
             f"\nqueried timestamps: {query_ts}"
             f"\nloaded timestamps: {loaded_ts}"
             f"\nvideo: {video_path}"
@@ -248,7 +249,11 @@ def decode_video_frames_torchvision(
     # convert to the pytorch format which is float32 in [0,1] range (and channel first)
     closest_frames = closest_frames.type(torch.float32) / 255
 
-    assert len(timestamps) == len(closest_frames)
+    if len(timestamps) != len(closest_frames):
+        raise FrameTimestampError(
+            f"Number of retrieved frames ({len(closest_frames)}) does not match "
+            f"number of queried timestamps ({len(timestamps)})"
+        )
     return closest_frames
 
 
@@ -353,11 +358,12 @@ def decode_video_frames_torchcodec(
     min_, argmin_ = dist.min(1)
 
     is_within_tol = min_ < tolerance_s
-    assert is_within_tol.all(), (
+    if not is_within_tol.all():
+        raise FrameTimestampError(
             f"One or several query timestamps unexpectedly violate the tolerance ({min_[~is_within_tol]} > {tolerance_s=})."
-        "It means that the closest frame that can be loaded from the video is too far away in time."
+            " It means that the closest frame that can be loaded from the video is too far away in time."
-        "This might be due to synchronization issues with timestamps during data collection."
+            " This might be due to synchronization issues with timestamps during data collection."
-        "To be safe, we advise to ignore this item during training."
+            " To be safe, we advise to ignore this item during training."
             f"\nqueried timestamps: {query_ts}"
             f"\nloaded timestamps: {loaded_ts}"
             f"\nvideo: {video_path}"