Document ModelOpt W4A16 NVFP4 Marlin path

[MerkyorLynn]

Summary

Document the Marlin path for ModelOpt W4A16 NVFP4 MoE checkpoints, and make
the Marlin NVFP4 log message accurate for weight-only NVFP4.

Why

ModelOpt mixed-precision MoE checkpoints can mark expert layers as
W4A16_NVFP4. This is a weight-only NVFP4 format: weights are 4-bit NVFP4 but
activations stay fp16/bf16. On this path Marlin is the correct backend; the
current warning implies Marlin is only used because the GPU lacks native FP4,
which can be misleading on GPUs with native FP4 support.

The docs now list W4A16_NVFP4 and MIXED_PRECISION, and include an explicit
Marlin serve example with --linear-backend marlin and --moe-backend marlin
for reproducible debugging and benchmarking.

Duplicate-work check

Searched open PRs for ModelOpt W4A16 NVFP4 Marlin. Related but not duplicate:

• b12x nvfp4 w4a16 use a16 fix #43929, [B12x][NVFP4] Add W4A16 support to FlashInfer SM12x MoE wrapper #43341, [B12x] W4A16 NVFP4 support + Nemotron-3.5 / Qwen3.5 fixes #43333, [Quantization][ModelOpt] W4A16 NVFP4 fused MoE + --override-activation-dtype flag #42428, [Quantization] Add ModelOpt FP8/NVFP4 weight-only embedding methods #42791, Pad FP4 Marlin weights to valid thread tiles #43806, and [Quant][Triton] Support modelopt_mixed on SM80/SM86 (Ampere) #42610 cover W4A16 /
  NVFP4 implementation details, backend support, or related quantization code.

This PR does not add a new W4A16 backend. It is limited to documentation and an
accuracy fix for the Marlin NVFP4 warning message.

Validation

• uv run --no-sync --python 3.12 python -m py_compile vllm/model_executor/kernels/linear/nvfp4/marlin.py
• git diff --check

No GPU validation was run for this docs/log-message patch.

AI assistance was used to prepare this patch.

[claude]

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[mergify]

Documentation preview: https://vllm--44672.org.readthedocs.build/en/44672/

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Avoid globally pinning Marlin for mixed FP8 layers

In the MIXED_PRECISION case described above where some non-MoE LinearBase layers are FP8, this global flag also forces those FP8 layers through MarlinFP8ScaledMMLinearKernel; choose_scaled_mm_linear_kernel filters by _get_linear_backend(), and that Marlin FP8 kernel rejects compute capability >= 89 unless VLLM_TEST_FORCE_FP8_MARLIN is set, so the documented SM120 command can fail at startup before reaching the W4A16 MoE path. Prefer only pinning --moe-backend marlin for these checkpoints, or document the extra env/caveat when FP8 linear layers are present.

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[MerkyorLynn]

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