[Bugfix] fix qwen3 moe fp8 accuracy issue

[jinzhen-lin]

Fix #22881 .
The origin issue is introduced by #22017 . The gate layer is initilized with fp8 quantization, but the origin weight is bf16.

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.

💬 Join our developer Slack at https://slack.vllm.ai to discuss your PR in #pr-reviews, coordinate on features in #feat- channels, or join special interest groups in #sig- channels.

Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which starts running only a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of those by going to your fastcheck build on Buildkite UI (linked in the PR checks section) and unblock them. If you do not have permission to unblock, ping simon-mo or khluu to add you in our Buildkite org.

Once the PR is approved and ready to go, your PR reviewer(s) can run CI to test the changes comprehensively before merging.

To run CI, PR reviewers can either: Add ready label to the PR or enable auto-merge.

🚀

[gemini-code-assist]

Code Review

This pull request addresses an FP8 accuracy issue for MoE models like Qwen3. The root cause was that layers intended to be skipped were being quantized because the configuration key modules_to_not_convert was not being checked. The fix correctly adds a fallback to check for this key if ignored_layers is not found or is empty. This change ensures compatibility with Hugging Face's quantization configuration format and resolves the accuracy problem. The implementation is correct and well-targeted.

[yewentao256]

Could you also add some lm-eval results to show the problem has already fixed?

[simon-mo]

verified locally as well. merging.

[simon-mo]

qwen3-30b-fp8

Before

+ curl http://localhost:8000/v1/completions -H 'Content-Type: application/json' -d '{
        "prompt": "What is the capital of France?",
        "max_tokens": 20,
        "temperature": 0
    }'
{"id":"cmpl-f570666360b0494faee6c8390c923980","object":"text_completion","created":1755391260,"model":"/mnt/localdisk/qwen3-30b-fp8","choices":[{"index":0,"text":"!!!!!!!!!!!!!!!!!!!!","logprobs":null,"finish_reason":"length","stop_reason":null,"prompt_logprobs":null}],"service_tier":null,"system_fingerprint":null,"usage":{"prompt_tokens":7,"total_tokens":27,"completion_tokens":20,"prompt_tokens_details":null},"kv_transfer_params":null}+ echo ''

After

+ curl http://localhost:8000/v1/completions -H 'Content-Type: application/json' -d '{
        "prompt": "What is the capital of France?",
        "max_tokens": 20,
        "temperature": 0
    }'
{"id":"cmpl-9b06470dce2d4f7fb42a946bbb4d2725","object":"text_completion","created":1755391074,"model":"/mnt/localdisk/qwen3-30b-fp8","choices":[{"index":0,"text":" The capital of France is Paris. Paris has been the capital since the 3rd century and is","logprobs":null,"finish_reason":"length","stop_reason":null,"prompt_logprobs":null}],"service_tier":null,"system_fingerprint":null,"usage":{"prompt_tokens":7,"total_tokens":27,"completion_tokens":20,"prompt_tokens_details":null},"kv_transfer_params":null}+ echo ''