enable flashinfer moe kernel for DP + EP

[czhu-cohere]

Purpose

Previously the BF16 flashinfer moe kernel is disabled when dp > 1. I think the kernel itself should be able to support it, we just need to enable on the vLLM side.

Test Plan

pytest tests/kernels/moe/test_unquantized_backend_selection.py
run gsm8k with bf16 qwen 3a30b on 2xB200 DP2 EP2 and compare the result with different moe backend.

server command

# triton/default backend
vllm serve Qwen/Qwen3-30B-A3B \
  --data-parallel-size 2 \
  --enable-expert-parallel \
  --trust-remote-code \
  --port 8000

# flashinfer cutlass
VLLM_USE_FLASHINFER_MOE_FP16=1 VLLM_FLASHINFER_MOE_BACKEND=throughput vllm serve Qwen/Qwen3-30B-A3B \
  --data-parallel-size 2 \
  --enable-expert-parallel \
  --trust-remote-code \
  --port 8000

# flashinfer trtllm
VLLM_USE_FLASHINFER_MOE_FP16=1 VLLM_FLASHINFER_MOE_BACKEND=latency vllm serve Qwen/Qwen3-30B-A3B \
  --data-parallel-size 2 \
  --enable-expert-parallel \
  --trust-remote-code \
  --port 8000

test command

python -m lm_eval \
  --model local-completions \
  --model_args "model=Qwen/Qwen3-30B-A3B,base_url=http://localhost:8000/v1/completions,num_concurrent=128,max_retries=5,tokenized_requests=False,tokenizer=Qwen/Qwen3-30B-A3B" \
  --tasks gsm8k_cot \
  --batch_size auto \
  --log_samples \
  --output_path /tmp/lm_eval_qwen_dp2_ep

Test Result

Backend	flexible-extract	stderr
Triton	0.8893	±0.0086
FlashInfer CUTLASS (throughput)	0.8961	±0.0084
FlashInfer TRTLLM (latency)	0.8976	±0.0083

pytest tests/kernels/moe/test_unquantized_backend_selection.py pass

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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 enables the FlashInfer CUTLASS MoE kernel for configurations using both Data Parallelism (DP) and Expert Parallelism (EP). The changes involve removing the restriction that prevented this kernel from being selected when DP is active. While the logic change appears correct, there is a lack of corresponding test updates to validate this new capability, which is a significant concern for ensuring correctness.

[gemini-code-assist]

This change enables the FlashInfer CUTLASS MoE kernel for configurations with Data Parallelism (use_dp=True). However, the corresponding tests in tests/kernels/moe/test_unquantized_backend_selection.py have not been updated to reflect this. The existing test test_select_cuda_flashinfer_cutlass_backend explicitly sets use_dp=False and includes a comment stating that CUTLASS does not support DP. To ensure the correctness of this feature and prevent future regressions, please add a new test case that validates the behavior when use_dp=True.

[czhu-cohere]

added test