[bugfix]Prevent overwriting drafters lm-head and embed_tokens

[HF-001]

What this PR does / why we need it?

Some EAGLE3 drafters might have their own lm_head and/or embed_tokens layers. Existing codebase ignores this.By solving this problem, it can greatly improve acceptance rates. Refer to this pr in vllm: vllm-project/vllm#27737

Does this PR introduce any user-facing change?

How was this patch tested?

export CUDA_VISIBLE_DEVICES=0
export TP=1
export MODEL_PATH=/model/Llama-3.1-8B-Instruct
export MODEL_NAME=Llama-3.1-8B-Instruct
export PORT=10133
python3 -m vllm.entrypoints.openai.api_server --host 0.0.0.0 --port ${PORT} --dtype bfloat16 --model ${MODEL_PATH} --served-model-name ${MODEL_NAME} --tensor-parallel-size ${TP} --gpu-memory-utilization 0.85 --max-model-len 32768 --trust-remote-code --seed 42 --speculative_config '{"method":"eagle3","model":"/model/EAGLE3-LLaMA3.1-Instruct-8B","num_speculative_tokens":5,"draft_tensor_parallel_size":1}'

• vLLM version: v0.11.0
• vLLM main: vllm-project/vllm@83f478b

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[gemini-code-assist]

Code Review

This pull request aims to prevent overwriting the lm_head and embed_tokens of a drafter model if they are already initialized, which is particularly important for some EAGLE3 drafters. The changes correctly add checks for has_own_embed_tokens and has_own_lm_head flags before sharing these layers from the target model. However, I've identified a critical regression in the logic for handling lm_head for SpecDcodeType.EAGLE. The check that ensures the target model has an lm_head attribute before it's accessed has been removed, which will likely cause an AttributeError at runtime. My review includes a specific code suggestion to fix this issue.

[gemini-code-assist]

The logic for sharing lm_head for SpecDcodeType.EAGLE has been changed in a way that introduces a potential AttributeError. The assignments to self.model.lm_head are no longer guarded by hasattr(model, "lm_head"). If the target model does not have an lm_head attribute, this will cause a crash. The original logic was safer and should be restored.

Suggested change

	if hasattr(model, "lm_head"):
	logger.info("Loading EAGLE LM head weights from the target model.")
	if supports_multimodal(model):
	self.model.lm_head = model.get_language_model().lm_head
	else:
	self.model.lm_head = model.lm_head
	if hasattr(model, "lm_head"):
	logger.info("Loading EAGLE LM head weights from the target model.")
	if supports_multimodal(model):
	self.model.lm_head = model.get_language_model().lm_head
	else:
	self.model.lm_head = model.lm_head