fix: ERROR: 'CompletionUsage' object has no attribute 'get' (#1736)

### What problem does this PR solve?

_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)

rag/llm/chat_model.py

@@ -183,7 +183,7 @@ class BaiChuanChat(Base):
                 if resp.choices[0].finish_reason == "stop":
                     if not resp.choices[0].delta.content:
                         continue
-                    total_tokens = resp.usage.get('total_tokens', 0)
+                    total_tokens = resp.usage.total_tokens
                 if not resp.choices[0].delta.content:
                     continue
                 ans += resp.choices[0].delta.content