[python] Read LeRobot video datasets for training

[XiaoHongbo-Hope]

Purpose

Read video-backed LeRobot v3 table groups through PaimonLeRobotDataset.

Changes

• decode .video frame descriptors with TorchCodec or PyAV
• reuse bounded decoder caches per DataLoader worker
• seek PyAV from known keyframes for efficient and correct random rereads
• support existing Episode, delta-window, padding, transform, and return_uint8 behavior

TorchCodec is preferred; PyAV is the fallback.

Tests

82 LeRobot/video tests passed, including real MP4, partial delta windows, B-frame random rereads, optional-dependency gates, and spawn workers.