[Kernel] Use FlashInfer CUDA BGMV MoE Kernel for Multi-LoRA MoE dispatch

[taehokim20]

Co-authored-by: Claude

Purpose

This PR connects vLLM's MoE LoRA dispatch path to FlashInfer's BGMV MoE CUDA kernel (merged in flashinfer-ai/flashinfer#3249). When FlashInfer with BGMV MoE support is installed, vLLM uses it via the flashinfer.fused_moe API. The Triton SGMV kernel remains as a fallback, selectable via environment variables.

This is the recommended integration path: kernel lives in FlashInfer, vLLM consumes it through the existing FlashInfer interface.

Changes

• flashinfer.py: Add has_flashinfer_bgmv_moe() detection and lazy wrappers for bgmv_moe_shrink / bgmv_moe_expand
• punica_gpu.py:
  • Add CUDA/Triton backend selection via VLLM_MOE_LORA_BACKEND env vars (default: CUDA)
  • Add pre-allocated GPU buffer infrastructure (_init_moe_bgmv_buffers) for zero-allocation kernel calls
  • Add add_lora_fused_moe_cuda method that dispatches to FlashInfer's bgmv_moe_shrink / bgmv_moe_expand
  • Add cudagraph capture skip (MoE LoRA ops are not capture-safe)
  • Route add_lora_fused_moe between CUDA (FlashInfer) and Triton paths based on backend config

Related PRs

• [Kernel] Multi-LoRA MoE CUDA BGMV kernel #40670 - Original PR with kernel implementation directly in vLLM (superseded by this approach)
• Add BGMV MoE CUDA kernels for multi-LoRA flashinfer-ai/flashinfer#3249 - FlashInfer PR with the CUDA kernel (merged)

Test Plan

Correctness verified by running vLLM's existing MoE LoRA kernel tests with FlashInfer dispatch enabled:

FLASHINFER_DISABLE_VERSION_CHECK=1 VLLM_MOE_LORA_BACKEND=cuda \
  python -m pytest tests/lora/test_fused_moe_lora_kernel.py -v

Test Result

All 417 passed

Performance results are documented in the FlashInfer PR (flashinfer-ai/flashinfer#3249) and the original vLLM PR (#40670).

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[jeejeelee]

Does the latest FI release contain this kernel?

[jeejeelee]

I am curious if this kernel can beat _fused_moe_lora_small_batch_kernel

[taehokim20]

I found _fused_moe_lora_small_batch_kernel in #42290. I'll check it. Thank you for letting me know!

[taehokim20]

Does the latest FI release contain this kernel?

It's not included in the latest FI release (https://github.com/flashinfer-ai/flashinfer/releases/tag/v0.6.12). This kernel is merged in the late Friday to FlashInfer main. We can wait for the next FI release.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @taehokim20.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork