Arm backend: Decompose sub/add with alpha!=1

[Erik-Lundell]

This was previously not supported, causing crashes in quantization, and incorrect output in floating point.

cc @freddan80 @per @zingo @oscarandersson8218 @digantdesai

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/14932

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❌ 3 New Failures, 1 Pending, 1 Unrelated Failure

As of commit d28d93f with merge base 8fbc42c ():

NEW FAILURES - The following jobs have failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t ad520288fa4de9f516d170ae9f4fedfc7d9cd59757042979e2bfc6ac35d22bda /exec failed with exit code 1
• Test CUDA Builds / test-voxtral-cuda-e2e / linux-job (gh)
  RuntimeError: Command docker exec -t ac1944cb886158781372b02d19ee49b7d952777c1c6d02f98a8c2c6ee7761619 /exec failed with exit code 2
• trunk / test-arm-cortex-m-size-test (zephyr-preset) / linux-job (gh)
  RuntimeError: Command docker exec -t 3d2f32e31024d6fdb1b49b451d1d141bdf07f804aa2e5cb294ee8f3af52beab1 /exec failed with exit code 1

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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[zingo]

trunk / test-arm-cortex-m-size-test (zephyr-preset) / linux-job (push) was broken already by
#14570

[zingo]

@pytorchbot cherry-pick --onto release/1.0 -c fixnewfeature

[pytorchbot]

Cherry picking #14932

The cherry pick PR is at #14939 and it is recommended to link a fixnewfeature cherry pick PR with an issue. The following tracker issues are updated:

• [v1.0.0] Release Tracker #14288 (comment)

Details for Dev Infra team

Raised by workflow job