[ROCm][Perf][Kimi-K3] Accumulate the latent-MoE up-projection in aiter a16w16 atomic

[nsusanto]

Purpose

The sharded latent-MoE tail adds each rank's routed up-projection into the shared-expert partial with hidden_shard.addmm_(latent, up_proj_shard.T). On MI355X this is hipBLASLt Cijk_..._MT16x16x512, 8.57 us per layer at decode (M=16, N=896, K=3584). This PR runs it through aiter's gemm_a16w16_atomic with accumulate=True, which adds into hidden_shard (a column slice of [M, 7168]) in place: 4.35 us per layer.

Per layer-step, rank 0, 1k/1k c16 decode trace:

Kernel	Before (us)	After (us)
Cijk_..._MT16x16x512 (addmm_)	8.57	-
_gemm_a16_w16_atomic_kernel (16x32x128, split-K 14)	-	4.35

Depends on ROCm/aiter#5895 (accumulate=True);

Test Plan

lm_eval \
  --model local-completions \
  --model_args "model=/data/Kimi-K3,base_url=http://localhost:8001/v1/completions,num_concurrent=64,max_retries=3,tokenized_requests=False,tokenizer_backend=None,trust_remote_code=True" \
  --tasks gsm8k \
  --num_fewshot 5 \
  --seed 42 \
  --output_path /app/lm_eval_out --log_samples

Server command:

#!/bin/bash

export VLLM_ROCM_USE_AITER=1
export SAFETENSORS_FAST_GPU=1
# Must agree with VLLM_ROCM_USE_AITER_MOE_SITUV2_A8W4 (both 1 -> a8w4 routed experts).
export AITER_SITUV2_A8W4=1
export VLLM_ROCM_USE_AITER_MOE_SITUV2_A8W4=1
export AITER_BF16_FP8_MOE_BOUND=0
export VLLM_USE_BREAKABLE_CUDAGRAPH=0
export VLLM_USE_RUST_FRONTEND=1
vllm serve /data/Kimi-K3 \
  --trust-remote-code \
  --port 8001 \
  --moe-backend auto \
  --tensor-parallel-size 8 \
  --load-format auto \
  --gpu-memory-utilization 0.95 \
  --mm-encoder-tp-mode data \
  --max-num-batched-tokens 4096 \
  --enable-auto-tool-choice \
  --tool-call-parser kimi_k3 \
  --reasoning-parser kimi_k3 \
  --max-num-seqs 128 --enable-prefix-caching \
  --enable-return-routed-experts \
  --compilation-config '{"cudagraph_mode":"FULL_DECODE_ONLY","custom_ops":["+fused_rms_norm_gated"]}' \
  "$@"

Bench Command:

vllm bench serve --backend vllm --model /data/Kimi-K3 \
  --host localhost --port ${PORT:-8001} --endpoint /v1/completions \
  --dataset-name random --random-input-len 1024 --random-output-len 1024 \
  --random-range-ratio 0 --request-rate=inf --random-prefix-len 0 \
  --num-prompts 160 --max-concurrency 16 --ignore-eos --trust-remote-code

Test Results

baseline

using tuned kernel but with additional add kernel (SE + RE outputs)

using tuned kernel with addm fusion in a16w16 atomic

|Tasks|Version|     Filter     |n-shot|  Metric   |   |Value |   |Stderr|
|-----|------:|----------------|-----:|-----------|---|-----:|---|-----:|
|gsm8k|      3|flexible-extract|     5|exact_match|↑  |0.9636|±  |0.0052|
|     |       |strict-match    |     5|exact_match|↑  |0.9636|±  |0.0052|

Before:

============ Serving Benchmark Result ============
Successful requests:                     160       
Failed requests:                         0         
Maximum request concurrency:             16        
Benchmark duration (s):                  248.84    
Total input tokens:                      163840    
Total generated tokens:                  163840    
Request throughput (req/s):              0.64      
Output token throughput (tok/s):         658.42    
Peak output token throughput (tok/s):    688.00    
Peak concurrent requests:                32.00     
Total token throughput (tok/s):          1316.83   
---------------Time to First Token----------------
Mean TTFT (ms):                          478.38    
Median TTFT (ms):                        489.75    
P99 TTFT (ms):                           533.62    
-----Time per Output Token (excl. 1st token)------
Mean TPOT (ms):                          23.86     
Median TPOT (ms):                        23.83     
P99 TPOT (ms):                           24.20     
---------------Inter-token Latency----------------
Mean ITL (ms):                           23.83     
Median ITL (ms):                         23.86     
P99 ITL (ms):                            24.57     
==================================================

After:

============ Serving Benchmark Result ============
Successful requests:                     160       
Failed requests:                         0         
Maximum request concurrency:             16        
Benchmark duration (s):                  244.92    
Total input tokens:                      163840    
Total generated tokens:                  163840    
Request throughput (req/s):              0.65      
Output token throughput (tok/s):         668.97    
Peak output token throughput (tok/s):    704.00    
Peak concurrent requests:                32.00     
Total token throughput (tok/s):          1337.93   
---------------Time to First Token----------------
Mean TTFT (ms):                          485.18    
Median TTFT (ms):                        496.71    
P99 TTFT (ms):                           532.36    
-----Time per Output Token (excl. 1st token)------
Mean TPOT (ms):                          23.47     
Median TPOT (ms):                        23.46     
P99 TPOT (ms):                           23.79     
---------------Inter-token Latency----------------
Mean ITL (ms):                           23.44     
Median ITL (ms):                         23.47     
P99 ITL (ms):                            24.12     
==================================================

AI assistance (Claude Code) was used for this change. I personally reviewed every changed line and ran the tests above.

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