Enable transformers multimodal export flow in extension/llm

[larryliu0820]

See README.md for why we are doing this instead of inside optimum-executorch.

Basically we need to change code in transformers and also touching xnnpack.py and custom_sdpa.py in optimum-executorch, these changes should be upstreamed to these repos respectively. For now let's put it in ET so that we can quickly iterate on it.

Summary

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Test plan

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[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/13031

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❌ 7 New Failures

As of commit 0f241f7 with merge base bedce91 ():

NEW FAILURES - The following jobs have failed:

• Lint / lintrunner / linux-job (gh)
  >>> Lint for extension/llm/optimum/image_text_to_text.py:
• trunk / test-arm-cortex-m-size-test (bare_metal) / linux-job (gh)
  RuntimeError: Command docker exec -t 9269a86fbd8f68468283143c112f89b0caeefb79f3c62b425466ba576bee55d2 /exec failed with exit code 1
• trunk / test-huggingface-transformers (allenai/OLMo-1B-hf) / linux-job (gh)
  RuntimeError: Command docker exec -t 15e7de32c4ab6bf1d951804f76fe9f407602dfae865b12c6c2b3053e136a64f3 /exec failed with exit code 1
• trunk / test-huggingface-transformers (google/gemma-3-1b-it) / linux-job (gh)
  RuntimeError: Quantization 8da4w requires torchao >= 0.11.0. Please upgrade torchao.
• trunk / test-huggingface-transformers (HuggingFaceTB/SmolLM2-135M) / linux-job (gh)
  RuntimeError: Quantization 8da4w requires torchao >= 0.11.0. Please upgrade torchao.
• trunk / test-huggingface-transformers (meta-llama/Llama-3.2-1B) / linux-job (gh)
  RuntimeError: Command docker exec -t 55da96db737062e4550224076e838891bcc4fe414d3b8b69dfdb0ab37f5d5ec7 /exec failed with exit code 1
• trunk / test-huggingface-transformers (Qwen/Qwen3-0.6B) / linux-job (gh)
  RuntimeError: Command docker exec -t 7d15deb40b5a31e3eb7e139738fd59b1182c47fdc9ff892a105dc48c217a5acc /exec failed with exit code 1

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