fix(dataset): reload episodes metadata before batch video encoding

[daikw]

Summary

Fixes an IndexError that occurs when using --resume with --dataset.video_encoding_batch_size > 1 during dataset recording.

Problem

When resuming recording with batch encoding enabled, the in-memory self.meta.episodes dataset was not being updated with newly recorded episodes. This caused an IndexError when trying to access episode metadata during batch encoding:

IndexError: Invalid key: 2 is out of bounds for size 2

The issue occurred because:

1. New episodes were saved to parquet files
2. self.meta.total_episodes was updated
3. But self.meta.episodes (HF Dataset) remained stale
4. Batch encoding tried to access episodes beyond the original size

Solution

Reload the episodes metadata at the start of _batch_save_episode_video() to ensure all episode data is available before accessing episode indices.

This follows the same pattern already used in line 1199 where episodes are reloaded when switching to a new chunk/file.

Test Plan

The fix should be tested by:

1. Recording a dataset with batch encoding disabled or with small batch size
2. Resuming recording with --resume=true and larger --dataset.video_encoding_batch_size
3. Verifying that batch encoding completes without IndexError

Example command that previously failed:

uv run lerobot-record \
  --robot.type=so101_follower \
  --robot.port=/dev/ttyACM1 \
  --teleop.type=so101_leader \
  --teleop.port=/dev/ttyACM0 \
  --dataset.repo_id=test/dataset \
  --dataset.video_encoding_batch_size=10 \
  --resume=true

🤖 Generated with Claude Code

[Copilot]

Pull Request Overview

This PR fixes an IndexError that occurs when resuming dataset recording with batch video encoding enabled. The issue was caused by stale episode metadata not being synchronized with newly recorded episodes when using the --resume flag.

• Adds a metadata reload operation before batch video encoding
• Ensures self.meta.episodes contains all available episodes before accessing them by index
• Follows existing pattern used elsewhere in the codebase for metadata synchronization

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.