Make MLX work when model is loaded on one thread and executed on another

[metascroy]

No description provided.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/22294

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❌ 20 Pending, 1 Unclassified Failure

As of commit 8849b81 with merge base ff50edd ():

UNCLASSIFIED FAILURE - DrCI could not classify the following job because the workflow did not run on the merge base. The failure may be pre-existing on trunk or introduced by this PR:

• MLX / test-mlx-llm-offgraph (google/gemma-4-E2B-it, gemma4-e2b, gemma4, macos-15-xlarge) / test-mlx-llm-offgraph-gemma4-e2b (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1

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[digantdesai]

CI is not happy :P

[metascroy]

CI is not happy :P

trunk is not happy :(

A PR changed delegate constant tagging last week, which requires corresponding mutable buffer change: #22296