[Killer-Queue]

Overview

Fix Qwen3-Omni Thinker lm_head prefix propagation for the NVFP4
quantization path.

This mirrors the related fix merged in
vllm-omni#4528.

Claims

• Pass the lm_head prefix to ParallelLMHead.
• Prevent the BF16 lm_head from being incorrectly treated as an NVFP4
  layer.
• Fix the 1024-vs-2048 weight loading failure for Qwen3-Omni NVFP4
  checkpoints.

Validation

Local validation:

• git diff --check: passed.

External end-to-end validation was performed on a separate server with:

• vLLM 0.29.0
• vllm:v0.29.0
• RTX PRO 5000 72GB Blackwell GPU
• dizuo-ModelOpt-NVFP4 checkpoint

Before this change, loading failed with:

RuntimeError: The size of tensor a (1024) must match
the size of tensor b (2048)

After this change:

• The NVFP4 checkpoint loaded successfully.
• All checkpoint shards loaded successfully.
• Application startup complete was reached.
• /v1/models reported nvfp4.
• A 32-token completion returned valid Chinese text.

The external validation used the vLLM serving path with:

vllm serve <NVFP4_CHECKPOINT> \
  --served-model-name nvfp4 \
  --trust-remote-code \
  --max-model-len 32768 \
  --max-num-batched-tokens 32768 \
  --gpu-memory-utilization 0.92 \
  --block-size 128 \
  --enable-prefix-caching

Details

Qwen3MoeLLMForCausalLM constructed ParallelLMHead without passing its
prefix. As a result, the ModelOpt NVFP4 exclusion rule could not match
lm_head, and the BF16 head was incorrectly treated as an NVFP4 layer.

The fix passes maybe_prefix(prefix, "lm_head"), matching the existing
Qwen3 MoE implementation. The change is limited to one model constructor
and does not alter behavior for other models or unquantized checkpoints.

No additional unit test was added because this failure requires a real
NVFP4 checkpoint and quantized weight-loading path. The affected path was
validated end to end on the required GPU environment.

---

Pull Request Checklist

• I used vLLM's /pr-checklist skill.

• AI assistance was used during the creation of this PR.

• Design Fit: Minimizes impact on core components, reuses existing functionality, and justifies added complexity.

• Testing and Validation: Validates the change and ensures any added tests are meaningful and reliable, with CI coverage or documented CI resource constraints and validation performed outside CI.

• Code Quality and Style: Keeps code and comments clear and concise, and updates relevant documentation and examples.

• Pull Request Contents: Includes a brief summary and relevant links, supports claims with evidence, explains root causes and implementation trade-offs, and follows the contributing guide.

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