[compressed-tensors] Asymmetric support for MoE WNA16 marlin

[brian-dellabetta]

Purpose

Prior to this PR, asymmetric WNA16 quantization schemes for MoEs were not supported through the compressed-tensors quant method. This PR updates to remove the constraint.

Resolves vllm-project/llm-compressor#2628

Test Plan

Validated that W4A16_ASYM improves on wikitext PPL over W4A16 (symmetric) baseline.

checkpoint creation script:

from llmcompressor import model_free_ptq

MODEL_ID = "Qwen/Qwen3.5-35B-A3B"
SAVE_DIR = MODEL_ID.rstrip("/").split("/")[-1] + "-W4A16"

model_free_ptq(
    model_stub=MODEL_ID,
    save_directory=SAVE_DIR,
    scheme="W4A16", # or "W4A16_ASYM"
    ignore=[
        "lm_head",
        "re:.*mlp.gate$",
        "re:.*mlp.shared_expert_gate.*",
        "re:.*norm.*",
        "re:.*embed_tokens.*",
        "re:.*visual.*",
        "re:.*conv1d.*",
    ],
    max_workers=15,
    device="cuda:0",
)

Test Result

W4A16 baseline:
lm_eval --model vllm --model_args "pretrained=Qwen3.5-35B-A3B-W4A16,add_bos_token=True,gpu_memory_utilization=0.6,pipeline_parallel_size=2" --tasks wikitext --batch_size 1

Tasks	Version	Filter	n-shot	Metric		Value		Stderr
wikitext	2	none	0	bits_per_byte	↓	0.5625	±	N/A
		none	0	byte_perplexity	↓	1.4768	±	N/A
		none	0	word_perplexity	↓	8.0442	±	N/A

W4A16_ASYM improvement:
lm_eval --model vllm --model_args "pretrained=Qwen3.5-35B-A3B-W4A16-ASYM,add_bos_token=True,gpu_memory_utilization=0.6,pipeline_parallel_size=2" --tasks wikitext --batch_size 1

Tasks	Version	Filter	n-shot	Metric		Value		Stderr
wikitext	2	none	0	bits_per_byte	↓	0.5568	±	N/A
		none	0	byte_perplexity	↓	1.4710	±	N/A
		none	0	word_perplexity	↓	7.8768	±	N/A

---

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[brian-dellabetta]

I had set this up for internal review opening in vllm-project. Closing this now in favor of that one:

• [compressed-tensors] Asymmetric support for MoE WNA16 marlin vllm-project/vllm#41409