Arm backend: Add decomposition and test for masked_fill.Scalar

[emmakujala]

Add decomposition and test for masked_fill.Scalar-operator

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/12746

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✅ You can merge normally! (1 Unrelated Failure)

As of commit f4a4bdf with merge base b562f36 ():

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

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

• Apple / build-demo-ios / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 74

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[emmakujala]

@pytorchbot label "partner: arm"

[emmakujala]

@pytorchbot label "release notes: arm"

[emmakujala]

@pytorchbot label "ciflow/trunk"

[pytorch-bot]

To add these label(s) (ciflow/trunk) to the PR, please first approve the workflows that are awaiting approval (scroll to the bottom of this page).

This helps ensure we don't trigger CI on this PR until it is actually authorized to do so. Please ping one of the reviewers if you do not have access to approve and run workflows.

[pytorch-bot]

To add the ciflow label ciflow/trunk please first approve the workflows that are awaiting approval (scroll to the bottom of this page).

This helps ensure we don't trigger CI on this PR until it is actually authorized to do so. Please ping one of the reviewers if you do not have access to approve and run workflows.

[Sebastian-Larsson]

Unrelated CI failure. Approved