Arm backend: Fix numerical accuracy error.

[AdrianLundell]

A missing .graph caused numerical errors to show up as "AttributeError: 'GraphModule' object has no attribute 'output_node'"

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/13578

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❌ 2 Cancelled Jobs, 1 Unrelated Failure

As of commit 75648a4 with merge base d87766e ():

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• trunk / test-huggingface-transformers-coreml (qwen3-1.7b|xnnpack|--quantize) / macos-job (gh)
• trunk / test-models-macos-coreml (efficient_sam) / macos-job (gh)
  ##[error]The operation was canceled.

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-benchmark-app / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 65

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