Add LoRA multimethod export to CoreML static LLM export

[lucylq]

Add --adapter CLI for exporting LoRA adapters as separate methods in
a CoreML PTE. CoreML POSITIONAL weight sharing deduplicates base weights
across methods. Supports combination with --multifunction for
decode/prefill variants per adapter.

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 LoRA support to StaticAttention for split_mha=False #18345
• Refactor: extract _transform_eager_model() from CoreML export main() #18343

[pytorch-bot]

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❌ 2 New Failures, 2 Unrelated Failures

As of commit 7ef0005 with merge base dd7464a ():

NEW FAILURES - The following jobs have failed:

• pull / test-moshi-linux / linux-job (gh)
  RuntimeError: Command docker exec -t ffa0848000bdd5657285f8b2b3c28dada064fac5e3f1ef857177f44bf96e15b6 /exec failed with exit code 1
• pull / test-static-llama-qnn-linux (stories_260k_bc) / linux-job (gh)
  RuntimeError: Command docker exec -t 6b681f0ad9c0d7cf5fee079f84caf0d0294b11d01465e426e561b67c2f341824 /exec failed with exit code 1

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]

add methods for each lora

[lucylq]

add methods for each lora with separate prefill, decode

Not sure if this is how we want to do it, though.

[lucylq]

Not sure if we should add a method like this here. Or if we add, it should be more granular, like a list of lora methods.

[lucylq]

Exclude lora from quantization for now; this should probably be a config.

[lucylq]

actually I think we can keep it in, and exclude if necessary later?