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[Bug]: Loading Qwen3MoE using Transformers backend #19801

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@Odysseusq

Description

@Odysseusq

Your current environment

The output of python collect_env.py
Your output of `python collect_env.py` here

🐛 Describe the bug

vllm serve Qwen/Qwen3-30B-A3B --model-impl=transformers

Result in error:

File ".../vllm/model_executor/models/transformers.py", line 66, in vllm_flash_attention_forward
self_attn = attention_instances[module.layer_idx]
TypeError: 'NoneType' object is not subscriptable

which I believe is a unique bug related to MoE structure, as the dense models of Qwen3 can be served normally with transformers backend.

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