use a set instead of list for VAECache unprocessed file lookups

[bghira]

This pull request optimizes how the VAECache class handles and checks membership in the local_unprocessed_files list by introducing a cached set representation, which improves performance and ensures correctness when the file list changes. It also adds a unit test to verify this behavior.

Performance and correctness improvements:

• Added a cached set (_local_unprocessed_files_set) and its source tracker to VAECache, with a helper method _local_unprocessed_file_set() that keeps the set in sync with local_unprocessed_files and is used for faster membership checks. [1] [2]
• Updated methods (_reduce_bucket and process_buckets) to use the cached set for membership checks instead of the list, improving performance and correctness. [1] [2] [3]
• Reset the cached set and its source to None whenever discover_unprocessed_files updates the file list, ensuring the cache stays accurate.

Testing:

• Added a unit test test_local_unprocessed_file_set_tracks_reassigned_file_list to verify that the cached set properly tracks changes to local_unprocessed_files.

[Copilot]

Pull request overview

This pull request improves VAECache membership-check performance by introducing a cached set view of local_unprocessed_files, keeping it in sync when the underlying list is reassigned, and updating the hot-path checks to use the set. It also adds a unit test to verify that the cache refreshes correctly when the list object changes.

Changes:

• Added _local_unprocessed_files_set and _local_unprocessed_files_set_source plus _local_unprocessed_file_set() to cache a set derived from local_unprocessed_files.
• Updated _reduce_bucket() and process_buckets() to use the cached set for membership checks instead of the list.
• Added a unit test to ensure the cached set updates when local_unprocessed_files is reassigned.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
simpletuner/helpers/caching/vae.py	Adds and uses a cached set for fast membership checks against local_unprocessed_files, with cache reset on rediscovery.
tests/test_vae.py	Adds a unit test validating the cached set refreshes when the underlying list is replaced.

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