Release 0.8.1

Patch release. Fixes S3 dataset downloads and adds the frames-parquet video loader plus released v20 holdout dataset configs.

Changes

• Fix: S3 downloads — corrected a relative-import depth bug in dataset/download/fetch.py (.utils → ..utils) that made every S3-source download throw No module named 'gasbench.dataset.download.utils'. All S3 holdout downloads were broken before this.
• feat: frames-parquet video loader — load video datasets stored in frames-parquet format.
• Config: released v20 holdout datasets added to the dataset registry.
• Retagged content_category for a few datasets; audio config formatting.

Verified end-to-end: v21 holdout downloads across image/video/audio (S3) succeed on this build.