tests: skip flaky MPS tests on CI

[psychedelicious]

Summary

I don't know what changed but at some point recently, MPS partial model loading tests have been getting flakier and flakier in CI, to the point that today not a single one has finished (have tried many times). We are OOMing in the tests.

I tried a few different things (clear torch caches before and after every model caching/loading test, clear GC, call torch.synchronize everywhere). Nothing worked. To unblock dev I've just marked these tests to skip in CI.

It's weird. If a change in our codebase broke CI, we'd expect that PR's CI to have never passed, preventing the offending change from getting into the codebase. Maybe it's a GH runner issue or upstream dependency change.

Related Issues / Discussions

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QA Instructions

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Merge Plan

This needs to merge ASAP so that other changes that alter python code can pass CI and themselves get merged.

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)