Partial Loading PR1: Tidy ModelCache

[RyanJDick]

Summary

This PR tidies up the model cache code in preparation for further refactoring to support partial loading of models onto the GPU. These code changes should not change the functional behavior in any way.

Changes:

• Remove the ModelCacheBase class. ModelCache is the only implementation, so there is no benefit to the separate abstract class.
• Split CacheRecord and CacheStats out into their own files.
• Remove the ModelLocker class. This extra layer of indirection was not providing any benefit. Locking is now done directly with the ModelCache.
• Tidy up relative imports that were contributing to circular import issues.
• Pull the 'submodel' concern out of the ModelCache. The ModelCache should not need to be aware of the model manager submodel system.
• Delete unused properties from the ModelCache (e.g. .lazy_offloading, .storage_device, etc.)

QA Instructions

I ran smoke tests with a variety of SD1, SDXL and FLUX models. No change to behavior is expected.

Merge Plan

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)