[Bug]: When use OpenAI chat model , raise ERROR: 'CompletionUsage' object has no attribute 'get' #2948 (#2949)

[Bug]: When use OpenAI chat model , raise ERROR: 'CompletionUsage'
object has no attribute 'get' #2948

### What problem does this PR solve?

the detail of this PR is shown at
https://github.com/infiniflow/ragflow/issues/2948

### Type of change

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

Co-authored-by: Kevin Hu <kevinhu.sh@gmail.com>

rag/llm/chat_model.py

@@ -67,6 +67,7 @@ class Base(ABC):
                 if not resp.choices[0].delta.content:
                     resp.choices[0].delta.content = ""
                 ans += resp.choices[0].delta.content
+
                 total_tokens += 1
                 if not hasattr(resp, "usage") or not resp.usage:
                     total_tokens = (