Fix a bug in VolcEngine (#3658)

### What problem does this PR solve?

Fix a bug in VolcEngine  #3553

### Type of change

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

Co-authored-by: liuhua <10215101452@stu.ecun.edu.cn>

rag/llm/__init__.py

@@ -48,6 +48,7 @@ EmbeddingModel = {
     "BaiduYiyan": BaiduYiyanEmbed,
     "Voyage AI": VoyageEmbed,
     "HuggingFace": HuggingFaceEmbed,
+    "VolcEngine":VolcEngineEmbed,
 }
 
 CvModel = {