[wip] 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):

• [wip] Add CI test for CoreML LoRA multimethod export #18354
• -> [wip] Add --method argument to CoreML static LLM runner #18355
• Add LoRA multimethod export to CoreML static LLM export #18347
• Add skip_names and LoRALinear guard to replace_linear_with_split_linear #18346
• Add LoRA support to StaticAttention for split_mha=False #18345
• Refactor: extract _prepare_eager_model() from CoreML export main() #18343

[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 Pending, 2 Unrelated Failures

As of commit 0984bff with merge base dd7464a ():

NEW FAILURE - The following job has failed:

• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t 7c5e90696990e963f7dc23dd2e8875c4a15f117d9f1678c07c4ff17b91633b94 /exec failed with exit code 139

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

• pull / unittest-editable / windows / windows-job (gh) (matched win rule in flaky-rules.json)
  ##[error]The operation was canceled.

BROKEN TRUNK - The following job failed but was 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.

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[lucylq]

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