CI: diagnose AOTI hang on macOS — isolated test with native stack sampling

[SS-JIA]

Summary

The macOS unittest job has been timing out since the PyTorch pin was updated to 2.12. Three CI runs showed 38-42 minutes of complete silence after ~55% test completion, with faulthandler unable to fire (confirming the hang is in native C/C++ code, not Python).

This PR isolates the diagnosis:

• Disables all CI jobs except the macos unittest job
• Replaces the full test suite with a single AOTI test (test_llama3_2_text_decoder_aoti) run as an instrumented script
• Prints timestamps before/after each step (export, compile, load, run) to identify which AOTI step hangs
• Runs a background watchdog that uses macOS sample to capture native C/C++ call stacks every 60s, since faulthandler cannot see into native code that holds the GIL
• Sets PYTHONUNBUFFERED=1 to prevent pipe buffering

Test plan

• CI should show timestamped output for each AOTI step
• If the test hangs, the watchdog sample output will show the exact native function blocking
• If the test passes, we know this specific AOTI test is not the culprit and should try other AOTI tests

Co-Authored-By: Claude noreply@anthropic.com

[pytorch-bot]

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✅ You can merge normally! (2 Unrelated Failures)

As of commit ff048ff with merge base 88faab2 ():

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

BROKEN TRUNK - The following job failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• trunk / test-arm-backend-ethos-u (test_smaller_stories_llama) / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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