[ROCm][Bugfix] Make intermediate_pad TP-aware in rocm_aiter_fused_experts

[Rohan138]

Purpose

Fix the GSM8K accuracy regression on the AITER MoE path at TP>1 introduced by #42098. This is because FlyDSL and CKTile MoE follow different conventions for hidden_pad/intermediate_pad in AITER, and the aiter.fused_moe dispatcher uses FlyDSL on TP1 but CKTile on TP8. This will partially be fixed on the AITER side in 0.1.15 by ROCm/aiter#3401.

On openai/gpt-oss-120b BF16 at TP=8 on MI355X, lm_eval GSM8K
flexible-extract drops from 0.896 (v0.21.0) to 0.684 (v0.22.0).
Bisected to #42098 changing intermediate_pad in
rocm_aiter_fused_experts to intermediate_pad // 64 * 64 * 2.

Test Plan

# Serve TP=8
VLLM_ROCM_USE_AITER=1 vllm serve openai/gpt-oss-120b \
    --dtype auto -tp 8 --no-enable-prefix-caching \
    --disable-uvicorn-access-log \
    --attention-backend ROCM_AITER_UNIFIED_ATTN

# Throughput
vllm bench serve --model openai/gpt-oss-120b \
    --dataset-name random --ignore-eos \
    --random-input-len 1024 --random-output-len 1024 \
    --max-concurrency 32 --num-prompts 64

# Accuracy
lm_eval --model local-completions \
    --model_args model=openai/gpt-oss-120b,base_url=http://localhost:8000/v1/completions,max_gen_toks=2048,num_concurrent=256,max_retries=10 \
    --tasks gsm8k --batch_size 1 --limit 250 --apply_chat_template

Test Result

openai/gpt-oss-120b BF16 on MI355X, 1k/1k random, mc=32:

Variant	TP=1 tput	TP=1 lmeval	TP=8 tput	TP=8 lmeval
Baseline (#42098)	3200 t/s	0.905	5392 t/s	0.684 ❌
TP-aware (PR)	3189	0.900	5168	0.892

TP=1 throughput within noise of the baseline; TP=8 accuracy is
recovered to v0.21.0 levels.

AI assistance (Claude) was used to bisect the regression, design and
verify this fix.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[dllehr-amd]

Thanks @Rohan138 for the quick fix. and the explanation for needing to adjust the intermediate padding! Approving, and we can tackle any additional issues when we bump Aiter again

[mergify]

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[AndreasKaratzas]

Adding it to milestone -- Fixes critical GPT OSS accuracy regression.