community[patch]: Invoke callback prior to yielding token (#18454)

## PR title
community[patch]: Invoke callback prior to yielding token

## PR message
- Description: Invoke callback prior to yielding token in _stream and
_astream methods in llms/baidu_qianfan_endpoint.
- Issue: https://github.com/langchain-ai/langchain/issues/16913
- Dependencies: None

libs/community/langchain_community/llms/baidu_qianfan_endpoint.py

@@ -213,9 +213,9 @@ class QianfanLLMEndpoint(LLM):
         for res in self.client.do(**params):
             if res:
                 chunk = GenerationChunk(text=res["result"])
-                yield chunk
                 if run_manager:
                     run_manager.on_llm_new_token(chunk.text)
+                yield chunk
 
     async def _astream(
         self,
@@ -228,7 +228,6 @@ class QianfanLLMEndpoint(LLM):
         async for res in await self.client.ado(**params):
             if res:
                 chunk = GenerationChunk(text=res["result"])
-
-                yield chunk
                 if run_manager:
                     await run_manager.on_llm_new_token(chunk.text)
+                yield chunk