[ExecuTorch][llm] Wire --use_moe_quantized_op through export pipeline and LLM config

[digantdesai]

Stack from ghstack (oldest at bottom):

• -> [ExecuTorch][llm] Wire --use_moe_quantized_op through export pipeline and LLM config #21126
• [ExecuTorch][llm] Add MoE quantized custom op documentation #21125
• [ExecuTorch][llm] Fuse w1+w3 into single GEMM in quantized_moe_ffn #21124
• [ExecuTorch][llm] Add numerical kernel tests for quantized_moe_ffn #21123
• [ExecuTorch][llm] Add source-transform and Meta-kernel tests for MoE op #21122
• [ExecuTorch][llm] Add MoE source transformation with INT4 quantized packing #21121
• [ExecuTorch][llm] Add Meta kernel and registration probe for quantized_moe_ffn #21120
• [ExecuTorch][llm] Add quantized_moe_ffn custom op with C++ kernel #21119

Add --use_moe_quantized_op to the argparse and LLM configuration paths, thread it through _get_source_transforms. The transform replaces eager MoE modules once through the common export pipeline and preserves shared-expert behavior.

Differential Revision: D102381996

[pytorch-bot]

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❗ 1 Active SEVs

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• Unrelated MacOS MPS test jobs time out (30 min) due to stale cpp_extension lock file poisoning self-hosted runners

❌ 9 New Failures, 1 Unrelated Failure, 4 Unclassified Failures

As of commit c60b650 with merge base bfed808 ():

NEW FAILURES - The following jobs have failed:

• Build Presets / apple (ios) / build (gh)
  Can't find 'action.yml', 'action.yaml' or 'Dockerfile' under '/Users/runner/work/executorch/executorch/test-infra/.github/actions/check-disk-space'. Did you forget to run actions/checkout before running your local action?
• Build Presets / apple (macos) / build (gh)
  Can't find 'action.yml', 'action.yaml' or 'Dockerfile' under '/Users/runner/work/executorch/executorch/test-infra/.github/actions/check-disk-space'. Did you forget to run actions/checkout before running your local action?
• Build Presets / apple (profiling) / build (gh)
  Can't find 'action.yml', 'action.yaml' or 'Dockerfile' under '/Users/runner/work/executorch/executorch/test-infra/.github/actions/check-disk-space'. Did you forget to run actions/checkout before running your local action?
• Build Presets / apple (pybind) / build (gh)
  Can't find 'action.yml', 'action.yaml' or 'Dockerfile' under '/Users/runner/work/executorch/executorch/test-infra/.github/actions/check-disk-space'. Did you forget to run actions/checkout before running your local action?
• pull / unittest / linux / linux-job (gh)
  extension/llm/custom_ops/test_quantized_moe.py::TestQuantizedMoeFfnOp::test_softmax
• pull / unittest / macos / macos-job (gh)
  extension/llm/custom_ops/test_quantized_moe.py::TestQuantizedMoeFfnOp::test_sigmoid_with_bias_route_scale_2p5
• pull / unittest-buck / linux / linux-job (gh)
  RuntimeError: Command docker exec -t c0d17d31b30aaa5ccd86f8637c4d04b5ebe1d2b89582ace0158c9ac62462a0c2 /exec failed with exit code 3
• pull / unittest-editable / linux / linux-job (gh)
  extension/llm/custom_ops/test_quantized_moe.py::TestQuantizedMoeFfnOp::test_softmax
• pull / unittest-editable / macos / macos-job (gh)
  extension/llm/custom_ops/test_quantized_moe.py::TestQuantizedMoeFfnOp::test_softmax

UNCLASSIFIED FAILURES - DrCI could not classify the following jobs because the workflow did not run on the merge base. The failures may be pre-existing on trunk or introduced by this PR:

• MLX / test-mlx-llm (unsloth/Llama-3.2-1B-Instruct, llama-1b, true, nvfp4, macos-14-xlarge) / test-mlx-llm-llama-1b-custom-nvfp4 (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  The process '/opt/homebrew/bin/git' failed with exit code 1
• MLX / test-mlx-qwen35-moe / test-mlx-qwen35-moe (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  examples/models/qwen3_5_moe/test_chunked_prefill.py::TestChunkedPrefill::test_chunked_prefill_matches_unchunked
• Test Metal Backend / test-model-metal-e2e (mistralai, Voxtral-Mini-3B-2507, non-quantized) / macos-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  Can't find 'action.yml', 'action.yaml' or 'Dockerfile' under '/Users/ec2-user/runner/_work/executorch/executorch/test-infra/.github/actions/check-disk-space'. Did you forget to run actions/checkout before running your local action?
• Test Metal Backend / test-model-metal-e2e (openai, whisper-large-v3-turbo, non-quantized) / macos-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  Can't find 'action.yml', 'action.yaml' or 'Dockerfile' under '/Users/ec2-user/runner/_work/executorch/executorch/test-infra/.github/actions/check-disk-space'. Did you forget to run actions/checkout before running your local action?

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest-buck / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 3

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