Partial Loading PR 3.5: Fix pre-mature model drops from the RAM cache

[RyanJDick]

Summary

This is an unplanned fix between PR3 and PR4 in the sequence of partial loading (i.e. low-VRAM) PRs. This PR restores the 'Current Workaround' documented in #7513. In other words, to work around a flaw in the model cache API, this fix allows models to be loaded into VRAM even if they have been dropped from the RAM cache.

This PR also adds an info log each time that this workaround is hit. In a future PR (#7509), we will eliminate the places in the application code that are capable of triggering this condition.

Related Issues / Discussions

• Partial Loading PR1: Tidy ModelCache #7492
• Partial Loading PR2: Add utils to support partial loading of models from CPU to GPU #7494
• Partial Loading PR3: Integrate 1) partial loading, 2) quantized models, 3) model patching #7500
• Model Cache API Flaw #7513

QA Instructions

• Set RAM cache limit to a small value. E.g. ram: 4
• Run FLUX text-to-image with the full T5 encoder, which exceeds 4GB. This will trigger the error condition.
• Before the fix, this test configuration would cause a KeyError. After the fix, we should see an info-level log explaining that the condition was hit, but that generation should continue successfully.

Merge Plan

No special instructions.

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)