[Bugfix][DeepSeek V4] Fix fused MTP RMSNorm dtype mismatch

[dongjibin1996]

Summary

• cast both fused MTP RMSNorm input paths to FP32 before the runtime branch merges
• keep position-zero masking and RMSNorm arithmetic in the existing FP32 compute type
• add a kernel correctness test covering the masked enorm path and both hnorm slots

Why

The two runtime branches previously assigned different inferred types to x: the masked tl.where path promoted it to FP32 while the hnorm path kept the input dtype. Triton requires local variables to have a consistent type when control-flow branches merge, so mixed-precision inputs could fail kernel compilation with a branch dtype mismatch.

Explicitly converting both loads to FP32 matches _rmsnorm_row, which already performs RMSNorm arithmetic in FP32. Results are still converted to the output pointer dtype at store time, so the output dtype and numerical behavior are preserved.

Duplicate-work check

I searched open vLLM PRs for fused_mtp_input_rmsnorm, MTP RMSNorm, and the dtype mismatch. PR #45240 introduced the shared fused kernel but does not contain this dtype fix or a standalone correctness test. I found no open PR addressing this issue.

Testing

• added tests/kernels/test_fused_mtp_input_rmsnorm.py to compare both fused outputs against a PyTorch RMSNorm reference
• python -m py_compile vllm/models/deepseek_v4/common/ops/fused_mtp_input_rmsnorm.py tests/kernels/test_fused_mtp_input_rmsnorm.py
• git diff --check

AI assistance

AI assistance was used to investigate the Triton type mismatch, prepare the patch, and draft the test and PR description. I reviewed the changed lines and test coverage.

[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.

[dongjibin1996]

Could a maintainer please add the verified label (or the appropriate CI authorization label) so pre-commit can run? The current pre-run-check failure is only the first-time contributor authorization gate; no pre-commit steps were executed.

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[dongjibin1996]

After re-evaluating this change against AGENTS.md, I am closing the PR. I cannot currently demonstrate the branch dtype failure on an upstream-supported configuration, and the added test does not establish a before-fail/after-pass regression there. Without that evidence, this is not sufficiently justified as a general vLLM bugfix. I will revisit it only if I can provide an upstream reproducer and targeted regression coverage.