Add --method argument to CoreML static LLM runner

[lucylq]

Allow specifying which method to execute from a multimethod PTE.
Defaults to "forward" for backward compatibility.

Authored with Claude.

[lucylq]

Stack from ghstack (oldest at bottom):

• Add CI test for CoreML LoRA multimethod export #18354
• -> Add --method argument to CoreML static LLM runner #18355
• Add LoRA multimethod export to CoreML static LLM export #18347

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18355

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❌ 1 New Failure, 2 Unrelated Failures

As of commit 45901a7 with merge base e90d3c8 ():

NEW FAILURE - The following job has failed:

• Lint / lintrunner-mypy (gh)
  >>> Lint for backends/arm/test/modules/test_static_cache.py:

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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[lucylq]

Note: this doesn't work with 'multifunction' which splits into prefill and decode.