Fix VolcEngine BUG (#1165)

### What problem does this PR solve?
- Fix a bug for VolcEngine
- After testing, the current VolcEngine configuration also supports the
Doubao series
_Briefly describe what this PR aims to solve. Include background context
that will help reviewers understand the purpose of the PR._

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

Co-authored-by: 海贼宅 <stu_xyx@163.com>

rag/llm/__init__.py

@@ -52,6 +52,7 @@ ChatModel = {
     "Xinference": XinferenceChat,
     "Moonshot": MoonshotChat,
     "DeepSeek": DeepSeekChat,
+    "VolcEngine": VolcEngineChat,
     "BaiChuan": BaiChuanChat,
     "MiniMax": MiniMaxChat,
     "Mistral": MistralChat