Fix the error that occurs when VLLM is called (#3076)

mem0/llms/vllm.py

@@ -1,6 +1,8 @@
 import json
 import os
 from typing import Dict, List, Optional
+from openai import OpenAI
+
 
 from mem0.configs.llms.base import BaseLlmConfig
 from mem0.llms.base import LLMBase