langchain/chat_models/openai.py/ChatOpenAI/_combine_llm_outputs
class ChatOpenAI:    def _combine_llm_outputs(self, llm_outputs: List[Optional[dict]]) -> dict:
        overall_token_usage: dict = {}
        for output in llm_outputs:
            if output is None:
                # Happens in streaming
                continue
            token_usage = output["token_usage"]
            for k, v in token_usage.items():
                if k in overall_token_usage:
                    overall_token_usage[k] += v
                else:
                    overall_token_usage[k] = v
        return {"token_usage": overall_token_usage, "model_name": self.model_name}