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
• -> 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
• Update lora def #18342

[pytorch-bot]

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