Expose the shardable unit of work behind recompute_stats: compute_dataset_episode_stats
computes per-episode stats for an episode subset, and aggregate_episode_stats merges the
concatenated shards (count-weighted) and writes stats.json. recompute_stats now composes
these, so single-process behavior is unchanged.
Add examples/dataset/slurm_recompute_stats.py, a datatrove compute/aggregate driver that
shards episodes across workers and is read-only safe (reference-copies the source when
--new-root is given). Most useful for the expensive image/video stats path.
CarolinePascal