[Kernel] Fuse MiniMax-M2 MoE align and sort fast path

[qianlihuang]

Purpose

Follow-up to vllm-project#43592.

This PR adds a MiniMax-M2-specific fast path for moe_align_block_size after the fused top-k routing + FP8 input quantization path.

-2.5us

Before

After
dp0_pp0_tp0_dcp0_ep0_rank0.1779958195845850305.pt.trace.json.gz

Test Plan

Test Result

============ Serving Benchmark Result ============
Successful requests:                     128       
Failed requests:                         0         
Maximum request concurrency:             16        
Benchmark duration (s):                  144.64    
Total input tokens:                      1053440   
Total generated tokens:                  131072    
Request throughput (req/s):              0.88      
Output token throughput (tok/s):         906.21    
Peak output token throughput (tok/s):    1088.00   
Peak concurrent requests:                26.00     
Total token throughput (tok/s):          8189.50   
---------------Time to First Token----------------
Mean TTFT (ms):                          1648.80   
Median TTFT (ms):                        1674.09   
P99 TTFT (ms):                           4200.47   
-----Time per Output Token (excl. 1st token)------
Mean TPOT (ms):                          16.05     
Median TPOT (ms):                        16.03     
P99 TPOT (ms):                           17.37     
---------------Inter-token Latency----------------
Mean ITL (ms):                           22.08     
Median ITL (ms):                         15.25     
P99 ITL (ms):                            235.85    
==================================================

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[Copilot]

Pull request overview

Adds a MiniMax-M2–specific CUDA fast path for moe_align_block_size to accelerate the align+sort stage after the fused top‑k routing + FP8 quant pipeline, and introduces a targeted correctness test that exercises the fast-path boundary.

Changes:

• Introduce minimax_moe_align_sort_kernel and dispatch to it when num_experts==256, topk==8, topk_ids is 2D, and topk_ids.numel() <= 1024 (no expert_map).
• Keep existing paths unchanged and fall back when the fast-path constraints aren’t met (e.g., m=129 for topk=8).
• Add a new unit test validating outputs vs. the torch reference for both fast-path and fallback boundary cases.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
csrc/moe/moe_align_sum_kernels.cu	Adds a MiniMax-M2 align+sort CUDA kernel and a guarded dispatch fast path in moe_align_block_size.
tests/kernels/moe/test_moe_align_block_size.py	Adds a correctness test covering MiniMax-M2 fast-path shapes and the fallback boundary.

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