Add ModelOpt W4A16 lm_head regression tests

[MerkyorLynn]

Summary

Add regression coverage for ModelOpt mixed-precision checkpoints that quantize
lm_head as W4A16_NVFP4.

Why

Some official ModelOpt NVFP4 MoE checkpoints include lm_head in
hf_quant_config.json with quant_algo = "W4A16_NVFP4". The LM head is a
ParallelLMHead, not a regular LinearBase, so it is useful to keep coverage
for this path separate from the generic W4A16 linear-layer tests.

This also adds Qwen3-MoE constructor coverage to ensure the model passes
quant_config into ParallelLMHead.

This PR is regression coverage only. It does not claim an end-to-end official
checkpoint load fix.

Duplicate-work check

Searched open PRs for ModelOpt W4A16 lm_head. Related but not duplicate:

• [Model] Nemotron-H 3.5: quantized LM head and MTP compressed-tensors fix #43342 covers Nemotron-H quantized LM head / MTP compressed-tensors handling.

This PR is scoped to ModelOpt mixed-precision W4A16_NVFP4 dispatch for
ParallelLMHead and Qwen3-MoE constructor coverage.

Validation

• uv run --no-sync --python 3.12 python -m py_compile tests/quantization/test_modelopt.py tests/model_executor/test_qwen3_5_quantization.py
• git diff --check

Targeted pytest was not run locally because this macOS checkout does not have
the vLLM test environment (pytest/torch) installed.

AI assistance was used to prepare this patch.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Mock Marlin support before instantiating W4A16 method

This new parametrized test runs unconditionally, but get_quant_method() constructs ModelOptNvFp4W4A16LinearMethod, whose constructor directly instantiates MarlinNvFp4LinearKernel; that kernel asserts is_fp4_marlin_supported() (current_platform.is_cuda() and capability >= 75). In CPU-only or unsupported-GPU test jobs this fails with an AssertionError, unlike the adjacent NVFP4 tests that patch kernel selection. Patch the Marlin kernel/support check here or skip the test when FP4 Marlin is unavailable.

Useful? React with 👍 / 👎.

[MerkyorLynn]

Hi maintainers, this is ready for review. Could you please add verified or ready if the scope looks appropriate? Thanks!

[MerkyorLynn]

Hi maintainers, quick clarification: the failing pre-run-check/RTD status appears to be the new-contributor gate rather than a docs/build failure. DCO passes, and this PR is intentionally scoped small.

Could a maintainer please review the scope and add verified or ready if it looks appropriate? Happy to revise or close if this does not fit vLLM’s contribution direction.