Fix: duplicate output by async_chat_streamly (#11842)

### What problem does this PR solve?

Fix: duplicate output by async_chat_streamly
Refact: revert manual modification

### Type of change

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

---------

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

rag/llm/chat_model.py

@@ -146,7 +146,6 @@ class Base(ABC):
             request_kwargs["stop"] = stop
 
         response = await self.async_client.chat.completions.create(**request_kwargs)
-
         async for resp in response:
             if not resp.choices:
                 continue
@@ -161,7 +160,6 @@ class Base(ABC):
             else:
                 reasoning_start = False
                 ans = resp.choices[0].delta.content
-
             tol = total_token_count_from_response(resp)
             if not tol:
                 tol = num_tokens_from_string(resp.choices[0].delta.content)
@@ -193,7 +191,7 @@ class Base(ABC):
             except Exception as e:
                 e = await self._exceptions_async(e, attempt)
                 if e:
-                    yield e
+                    yield e                    
                     yield total_tokens
                     return