add cot_prompt in vllm

[HERIUN]

Add Cot prompt argument. #8

cot prompt is appended after example.

        # Actual example
        content = example + cot_prompt if cot_prompt is not None else example

        if use_chat_template:
            examples.append({"role": "user", "content": content})
        else:
            examples.append(content)

[HuggingFaceDocBuilderDev]

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