Run a subset of backend test suite on pull

[GregoryComer]

Summary

Run a subset of operator and model tests on pull. This should help keep these jobs healthy and give better signal.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18426

• 📄 Preview Python docs built from this PR

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❌ 1 Cancelled Job, 2 Unrelated Failures

As of commit f38ea38 with merge base 0ffca95 ():

CANCELLED JOB - The following job was cancelled. Please retry:

• pull / unittest-editable / macos / macos-job (gh)
  ##[error]The operation was canceled.

BROKEN TRUNK - The following jobs failed but were present on the merge base:

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

• pull / unittest / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[github-actions]

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