style: fix typo and format code (#2618)

### What problem does this PR solve?

- Fix typo
- Remove unused import
- Format code

### Type of change

- [x] Other (please describe): typo and format

rag/llm/chat_model.py

@@ -20,7 +20,6 @@ from abc import ABC
 from openai import OpenAI
 import openai
 from ollama import Client
-from volcengine.maas.v2 import MaasService
 from rag.nlp import is_english
 from rag.utils import num_tokens_from_string
 from groq import Groq
@@ -29,6 +28,7 @@ import json
 import requests
 import asyncio
 
+
 class Base(ABC):
     def __init__(self, key, model_name, base_url):
         self.client = OpenAI(api_key=key, base_url=base_url)