[Compile] Fix import error of vllm._C

[ZJY0516]

Purpose

PR #24673 removes nvfp4_blockwise_moe_kernel.cu from VLLM_EXT_SRC, which will cause an import error when using vllm built from source.

  File "/home/zjy/code/vllm-src/vllm/platforms/cuda.py", line 18, in <module>
    import vllm._C  # noqa
    ^^^^^^^^^^^^^^
ImportError: /home/zjy/code/vllm-src/vllm/_C.abi3.so: undefined symbol: _Z20cutlass_fp4_group_mmRN2at6TensorERKS0_S3_S3_S3_S3_S3_S3_S3_

Test Plan

Test Result

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
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[gemini-code-assist]

Code Review

This pull request correctly fixes a build issue that was causing an ImportError due to an undefined symbol. The error was introduced in a previous pull request that removed nvfp4_blockwise_moe_kernel.cu from the list of compiled sources. This change re-adds the file to CMakeLists.txt, which resolves the issue. The fix is straightforward and effective. I have reviewed the change and found no issues.

[ProExpertProg]

We should instead just guard its use in torch_bindings.cpp on the arch supporting fp4 (and the cudaversion)

[ProExpertProg]

It's added to sources here

[ZJY0516]

We should instead just guard its use in torch_bindings.cpp on the arch supporting fp4 (and the cudaversion)

I didn't seen similar pattern like this in torch_bindings.cpp so I directly changed CMakeLists.txt.

[ZJY0516]

@LucasWilkinson @tlrmchlsmth @ProExpertProg PTAL

[LucasWilkinson]

LGTM, thanks!

[ProExpertProg]

Thanks for reworking this

[ProExpertProg]

#26138 does it in a cleaner way actually

[DarkLight1337]

Closing as superseded by #26138