[Feature][Kernels]: Integrate FlashInfer MoE Fused Finalize

[robertgshaw2-redhat]

🚀 The feature, motivation and pitch

We are working on fusing all small ops in DSR1 and other popular models.

There are open work streams on a couple of these:

• RMSNorm + BlockFP8
• ROPE+KV Insert
• All Reduce + RMSNorm

One other one that is possible is fusing the MoE finalize reduction. Here is an op in FlashInfer:

• https://github.com/flashinfer-ai/flashinfer/blob/d42fb90ee59d269b77b218a93467f8af58756eba/flashinfer/comm/trtllm_ar.py#L349

• So this would fuse the application + reduction of the topk weights onto the shared and routed experts in MoE layers

• Here's an example trace

cc @ProExpertProg

Alternatives

none

Additional context

https://vllm-dev.slack.com/archives/C08NFPURQ1F/p1762802402502609

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