[Core ML] Add test for stateful model and fix output backings issue

[cymbalrush]

Fixes a couple of issues

• Prewarms stateful model
• The prediction was run twice if output backings was used.
• Adds unit test for stateful model

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/5294

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❌ 3 New Failures, 1 Unrelated Failure

As of commit 835eeb5 with merge base cac2c05 ():

NEW FAILURES - The following jobs have failed:

• Apple / test-demo-ios / macos-job (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• trunk / test-models-macos (cmake, emformer_join, portable, macos-m1-stable, 90) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• trunk / test-models-macos (cmake, mul, portable, macos-m1-stable, 90) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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

• Apple / upload-frameworks-ios (gh) (detected as infra flaky with no log or failing log classifier)

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[cymbalrush]

@pytorchbot label ciflow/trunk

[cccclai]

Thanks for the fix! Does stateful model also work on old OS?

[facebook-github-bot]

@cccclai has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[cymbalrush]

Thanks for the fix! Does stateful model also work on old OS?

Unfortunately no, state ops were introduced in iOS 18.

[facebook-github-bot]

@cccclai merged this pull request in 08c8c6e.