community[patch]: Invoke callback prior to yielding token (#18449)

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

## PR message
- Description: Invoke callback prior to yielding token in _stream method
in chat_models/perplexity.
- Issue: https://github.com/langchain-ai/langchain/issues/16913
- Dependencies: None

libs/community/langchain_community/chat_models/perplexity.py

@@ -230,9 +230,9 @@ class ChatPerplexity(BaseChatModel):
             )
             default_chunk_class = chunk.__class__
             chunk = ChatGenerationChunk(message=chunk, generation_info=generation_info)
-            yield chunk
             if run_manager:
                 run_manager.on_llm_new_token(chunk.text, chunk=chunk)
+            yield chunk
 
     def _generate(
         self,