[Bug Fix] Fixes ngram spec decode bug introduced by vllm

[wxsIcey]

What this PR does / why we need it?

[Bug Fix] Fixes ngram spec decode bug introduced by vllm vllm-project/vllm#24986

Does this PR introduce any user-facing change?

N/A

How was this patch tested?

def main():
    prompts = [
        "The future of AI is",
    ]

    # Create a sampling params object.
    sampling_params = SamplingParams(temperature=0.8, top_p=0.95)
    # Create an LLM.
    llm = LLM(
            model="LLM-Research/Meta-Llama-3.1-8B-Instruct",
            tensor_parallel_size=1,
            speculative_config={
                "method": "ngram",
                "num_speculative_tokens": 5, # 每次最多推测 5 个 token
                "prompt_lookup_max": 4, # 最多使用 4 个 n-gram 进行匹配
            },
            enforce_eager=True,
        )

    # Generate texts from the prompts.
    outputs = llm.generate(prompts, sampling_params)
    print(f"Outputs: {outputs}")
    for output in outputs:
        prompt = output.prompt
        generated_text = output.outputs[0].text
        print(f"Prompt: {prompt!r}, Generated text: {generated_text!r}")

• vLLM version: v0.11.0rc3
• vLLM main: vllm-project/vllm@c9461e0

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