Make dataset classes drop-in compatible with lerobot's dataset factory

[AyushExel]

lerobot-train has no hook to swap its dataset class, so integrating the plugin previously required a user-side subclass shadowing absolute_to_relative_idx plus an adapter function for the constructor signature. This PR moves that interface responsibility into the plugin, where it belongs. After it, the entire training integration is:

import lerobot.datasets.factory as factory
from lerobot_lancedb import LeRobotLanceVideoDataset

factory.LeRobotDataset = LeRobotLanceVideoDataset  # done

if __name__ == "__main__":
    from lerobot.scripts.lerobot_train import main
    main()

Changes (both LeRobotLanceDataset and LeRobotLanceVideoDataset)

1. Upstream-compatible constructor order — repo_id first positional, root second, matching LeRobotDataset(repo_id, root=...) as the factory calls it. A positional local path is still detected (Path(arg).is_dir()) and treated as root, so pre-existing positional call sites keep working.
2. absolute_to_relative_idx = None class attribute — lance serves every frame by absolute dataset index; the upstream property exists only for episode-filtered parquet readers and would try to build one. Semantically correct for lance, and lets EpisodeAwareSampler wiring read it safely.
3. Factory kwargs tolerated — video_backend, download_videos, force_cache_sync, depth_output_unit accepted and ignored (documented as no-ops for Lance reads).

Verification

• 4-GPU accelerate launch run of lerobot-train (smolvla on the LIBERO lance dataset, --dataset.episodes filter active): trains normally, data_s=0.002.
• All four call styles tested: factory-style (repo_id, root=...), keyword root=, positional path, and factory-style with parquet-only kwargs.
• Test suite: 14 passed; test_subtask_index_round_trips errors identically on main under lerobot 0.6.0 (pre-existing meta.subtasks API drift, unrelated).

Context: this fell out of the end-to-end LIBERO example/blog (lancedb/training#6), where the user-side shim was the one wart in the migration story.

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