[Refactor] Small refactor for group topk

[yewentao256]

Purpose

Thanks for comments from @mgoin #30159 (review) and #29125 (review).

This PR fixes this.

The division could give us some performance benefits as well

Test

(wentao) wentao@dgxB200-09:~/vllm-source$ pytest tests/kernels/moe/test_grouped_topk.py 
================================= test session starts =================================
platform linux -- Python 3.12.3, pytest-9.0.1, pluggy-1.6.0
rootdir: /home/wentao/vllm-source
configfile: pyproject.toml
plugins: forked-1.6.0, anyio-4.11.0, timeout-2.4.0
collected 96 items                                                                    

tests/kernels/moe/test_grouped_topk.py ........................................ [ 41%]
........................................................                        [100%]

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
=========================== 96 passed, 2 warnings in 35.13s ===========================

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[gemini-code-assist]

Code Review

This pull request introduces a couple of refactorings in the grouped_topk CUDA kernel. The apply_scoring function is made more robust by explicitly handling SCORING_NONE and adding a static_assert for unsupported scoring functions, which is a good improvement. The main change is a performance optimization in group_idx_and_topk_idx_kernel where a division is moved out of a loop. While this improves performance, I've raised a concern about the change in the order of floating-point operations, which could potentially affect numerical precision and determinism. The removal of a commented-out line in a test file is a minor but good cleanup.

[mgoin]

Sweet, thank you!