community[patch]: callback before yield for _stream/_astream (#17907)

- Description: callback on_llm_new_token before yield chunk for
_stream/_astream for some chat models, make all chat models in a
consistent behaviour.
- Issue: N/A
- Dependencies: N/A

libs/community/langchain_community/chat_models/gpt_router.py

@@ -325,13 +325,13 @@ class GPTRouter(BaseChatModel):
                 chunk.data, default_chunk_class
             )
 
-            yield chunk
-
             if run_manager:
                 run_manager.on_llm_new_token(
                     token=chunk.message.content, chunk=chunk.message
                 )
 
+            yield chunk
+
     async def _astream(
         self,
         messages: List[BaseMessage],
@@ -358,13 +358,13 @@ class GPTRouter(BaseChatModel):
                 chunk.data, default_chunk_class
             )
 
-            yield chunk
-
             if run_manager:
                 await run_manager.on_llm_new_token(
                     token=chunk.message.content, chunk=chunk.message
                 )
 
+            yield chunk
+
     def _create_message_dicts(
         self, messages: List[BaseMessage], stop: Optional[List[str]]
     ) -> Tuple[List[Dict[str, Any]], Dict[str, Any]]: