Fix MoE reduce-scatterv eligibility check

[samuellees]

Summary

• Enable DP reduce-scatterv only when the TP group size matches the attention-DP split count.
• Fall back to all-reduce plus DP scatter when the group sizes do not match.
• Add unit tests covering both valid and fallback paths

Checklist

• Format your code according to the Format code with pre-commit.
• Add unit tests according to the Run and add unit tests.
• Update documentation according to Write documentations.
• Provide accuracy and speed benchmark results according to Test the accuracy and Benchmark the speed.
• Follow the SGLang code style guidance.

Review and Merge Process

1. Ping Merge Oncalls to start the process. See the PR Merge Process.
2. Get approvals from CODEOWNERS and other reviewers.
3. Trigger CI tests with comments or contact authorized users to do so.
   • Common commands include /tag-and-rerun-ci, /tag-run-ci-label, /rerun-failed-ci
4. After green CI and required approvals, ask Merge Oncalls or people with Write permission to merge the PR.

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CI States

Latest PR Test (Base): ✅ Run #30369958254
Latest PR Test (Extra): ✅ Run #30450094634

[gemini-code-assist]

Caution

The consumer version of Gemini Code Assist on GitHub has been sunset. All code review activity has officially ceased.

[ispobock]

/tag-and-rerun-ci