Fix gpt oss triton moe

[Rohan138]

Purpose

Revert/fix #725 to add back triton moe support for default A16W4 gpt-oss weights. Also fix padding for A16W4 CK backend.

Test Plan

Test Result

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Essential Elements of an Effective PR Description Checklist

• 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.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[Rohan138]

Repro script:

export VLLM_USE_AITER_UNIFIED_ATTENTION=1 VLLM_ROCM_USE_AITER=1 VLLM_ROCM_USE_AITER_MHA=0
export VLLM_ROCM_USE_AITER_FUSED_MOE_A16W4=1

MODEL=openai/gpt-oss-120b
# MODEL=amd/gpt-oss120b-w-mxfp4-a-fp8

vllm serve $MODEL -tp 8 --async-scheduling --swap-space 16 --no-enable-prefix-caching --disable-log-requests --disable-uvicorn-access-log --trust-remote-code --block-size 64 --compilation-config '{"cudagraph_mode":"FULL_AND_PIECEWISE"}'

# lm-eval
lm_eval --model local-completions --model_args model=$MODEL,base_url=http://0.0.0.0:8000/v1/completions,num_concurrent=256,max_retries=10,max_gen_toks=2048 --batch_size auto --tasks gsm8k --num_fewshot 5 --limit 200  --output_path . --apply_chat_template 2>&1 | tee -a eval.log

MI300 Triton w4a16

Tasks	Version	Filter	n-shot	Metric		Value		Stderr
gsm8k	3	flexible-extract	5	exact_match	↑	0.960	±	0.0139
		strict-match	5	exact_match	↑	0.665	±	0.0335

MI350 Triton w4a8

Tasks	Version	Filter	n-shot	Metric		Value		Stderr
gsm8k	3	flexible-extract	5	exact_match	↑	0.945	±	0.0162
		strict-match	5	exact_match	↑	0.665	±	0.0335

MI350 Triton w4a16

Tasks	Version	Filter	n-shot	Metric		Value		Stderr
gsm8k	3	flexible-extract	5	exact_match	↑	0.96	±	0.0139
		strict-match	5	exact_match	↑	0.65	±	0.0338

MI350 CK w4a16

Tasks	Version	Filter	n-shot	Metric		Value		Stderr
gsm8k	3	flexible-extract	5	exact_match	↑	0.960	±	0.0139
		strict-match	5	exact_match	↑	0.705	±	0.0323