fix: video chunk no corresponding data chunk

[bingogome]

What this does

This PR fix an error of meta file mapping.

The old aggregate.py will produce a merged dataset that has (as an example, the number of video files does not match the number of data files, and the meta has an incorrect mapping):

data/chunk-000/file-000.parquet
videos/observation.images.left/chunk-000/file-000.mp4
videos/observation.images.left/chunk-000/file-001.mp4

This will bug out when using data conversion tools and reports:

"/.../convert_dataset_v30_to_v21.py", line 175, in convert_data
raise FileNotFoundError(f"Expected source parquet file not found: {source_path}")
FileNotFoundError: Expected source parquet file not found: .../data/chunk-000/file-001.parquet

The above happens when you merge dataset A and B, where A has

data/chunk-000/file-000.parquet
data/chunk-000/file-001.parquet
videos/observation.images.left/chunk-000/file-000.mp4
videos/observation.images.left/chunk-000/file-001.mp4

and B has

data/chunk-000/file-000.parquet
videos/observation.images.left/chunk-000/file-000.mp4

[Copilot]

Pull Request Overview

This PR fixes a metadata mapping issue in dataset aggregation that caused misalignment between video files and their corresponding data files. The fix introduces a data_mapping dictionary to track the actual destination chunk/file indices when data files are merged, ensuring metadata correctly references the consolidated data files.

Key Changes:

• Added data mapping tracking to ensure metadata points to correct data file locations after aggregation
• Modified append_or_create_parquet_file to return destination chunk/file indices
• Updated update_meta_data to use the mapping instead of simple offset arithmetic

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