Bug fix for volcengine (#909)

### What problem does this PR solve?
Bug fixes for the VolcEngine

- Bug fix for front-end configuration code of VolcEngine

- Bug fix for tokens counting logic of VolcEngine


### 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/chat_model.py

@@ -359,6 +359,7 @@ class VolcEngineChat(Base):
         if system:
             history.insert(0, {"role": "system", "content": system})
         ans = ""
+        tk_count = 0
         try:
             req = {
                 "parameters": {
@@ -376,10 +377,9 @@ class VolcEngineChat(Base):
                 if not resp.choices[0].message.content:
                     continue
                 ans += resp.choices[0].message.content
-                yield ans
                 if resp.choices[0].finish_reason == "stop":
-                    yield resp.usage.total_tokens
-
+                    tk_count = resp.usage.total_tokens
+                yield ans
         except Exception as e:
             yield ans + "\n**ERROR**: " + str(e)
-        yield 0
+        yield tk_count