Enhance the robustness of the code (#1879)

### What problem does this PR solve?

Enhance the robustness of the code

### Type of change

- [x] Refactoring

---------

Co-authored-by: Zhedong Cen <cenzhedong2@126.com>

rag/llm/chat_model.py

@@ -72,7 +72,7 @@ class Base(ABC):
                         + num_tokens_from_string(resp.choices[0].delta.content)
                     )
                     if not hasattr(resp, "usage") or not resp.usage
-                    else resp.usage["total_tokens"]
+                    else resp.usage.get("total_tokens",total_tokens)
                 )
                 if resp.choices[0].finish_reason == "length":
                     ans += "...\nFor the content length reason, it stopped, continue?" if is_english(