Rocm aiter fused qknorm ar v2

[pbkowalski]

PR vllm-project#42602 was authored on May 14 against MiniMaxQKNormPass, an Inductor FX fusion pass that pattern-matched the forward_qk graph at compile time and rewrote it to call the AITER op. That entire mechanism was deleted from upstream main by PR vllm-project#43410: the FX fusion pass was replaced with a manual fusion dispatcher — a Torch custom op (torch.ops.vllm.minimax_qk_norm_fusion, implemented by _minimax_qk_norm_fusion in rms_norm_tp.py) called directly from

MiniMaxText01RMSNormTP.forward_qkv in minimax_m2.py. The file PR vllm-project#42602 added (vllm/compilation/passes/fusion/minimax_qk_norm_fusion.py) no longer exists, and the pass config flag (fuse_minimax_qk_norm) no longer wires anything up.

This is a forward-port of vllm-project#42602's intent — dispatch ROCm QK-norm + AllReduce through AITER's custom_fused_qknorm_ar — onto the post-vllm-project#43410 manual-fusion architecture.

PR vllm-project#42602's pass capped ROCm at MAX_TOKEN_NUM = 80 because AITER's qknorm_allreduce_fusion_kernel_2stage has a kMaxBlocks = 80 hard cap in its C++ header (csrc/include/custom_all_reduce.cuh). The pass solved this at compile time: only piecewise sub-graphs whose compile-range end ≤ 80 were rewritten.

This PR reuses the CUDA gate (MINIMAX_QK_NORM_MAX_TOKEN_NUM = 2048), which does not match AITER's hard cap. In our MI355X test sweep this didn't surface because:

• Decode steps stayed under 80 tokens (max_concurrency=16 × num_speculative_tokens=3 + 1 = ~64).
• Prefill chunks of up to 16,384 tokens may have hit the AITER kernel above its cap, but we did not detect garbage output or hangs in the bench results or in lm_eval-style verification (we didn't run that).

Before merge, the gate should probably be tightened to:

ROCM_AITER_QKNORM_AR_MAX_TOKEN_NUM = 80 # AITER kMaxBlocks

and used in the new ROCm branch instead of MINIMAX_QK_NORM_MAX_TOKEN_NUM. Above 80 tokens, fall through to _minimax_qk_norm_fallback. This is the only meaningful behavioural divergence from the original PR's safety profile.

Things that did not change vs vllm-project#42602

• Kernel called: aiter.dist.device_communicators.custom_all_reduce.CustomAllreduce.custom_fused_qknorm_ar.
• Output contract: returns (q, k); v left to the caller's post-split.
• Honours --disable-custom-all-reduce, VLLM_ROCM_USE_AITER=0, missing-aiter, older-aiter.
• No new env vars, no model-file changes (we removed the minimax_m2_v3.py forward_qkv → forward_qk revert mount).
• CUDA behavior unchanged.

TL;DR

PR vllm-project#42602 was a compile-time FX pass; upstream then refactored MiniMax-M2 fusion into a runtime manual-dispatch op. This PR is the same kernel dispatch, ported into the new dispatcher, at one-fifth the diff size and with no lifecycle ownership of the AITER CustomAllreduce. The one caveat is that PR vllm-project#42602's stricter 80-token gate was not carried over and should be added before merging.

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[akii96]

The commit and change history was messy so I cherry picked your commit (with your authorship preserved) + added some fixes of my own on top! the new branch for this lives here -> https://github.com/akii96/vllm/tree/akii96/minimax-m2-fused-qknorm-aiter

I will run testing tomorrow morning before opening the new version of this PR upstream