Allow arbitrary shard iteration

[kralka]

Start migration to allow customizing shard info iteration.

[gemini-code-assist]

Summary of Changes

Hello @kralka, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This PR begins the migration towards more flexible shard iteration by introducing a new CachedShardInfoIterator class. This class centralizes logic for filtering, limiting, and shuffling shards, which was previously scattered. It also refactors the dataset iteration methods to leverage this new, more customizable iterator.

Highlights

• Introduced CachedShardInfoIterator: A new class that extends ShardInfoIterator to provide advanced control over shard iteration, including caching, filtering by shard_filter, limiting by shards count, limiting by custom_metadata_type_limit, and shuffling.
• Centralized Shard Selection Logic: The shard_paths_dataset method now utilizes CachedShardInfoIterator to handle all shard filtering and limiting, removing duplicated logic.
• Refactored Example Iteration: The as_numpy_iterator method has been updated to use CachedShardInfoIterator and delegates the core example processing to a new helper method, example_iterator_from_shard_iterator, improving modularity.

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[gemini-code-assist]

Code Review

This pull request introduces a CachedShardInfoIterator to centralize and customize shard iteration logic. This is a good refactoring that simplifies methods like shard_paths_dataset and as_numpy_iterator in DatasetIteration. My review includes a suggestion to improve the robustness of metadata handling in the new iterator and a note on some logging that was removed during the refactoring, which might be worth restoring for debuggability.