langchain/memory/kg.py/ConversationKGMemory/_get_prompt_output_key
class ConversationKGMemory:    def _get_prompt_output_key(self, outputs: Dict[str, Any]) -> str:
        """Get the output key for the prompt."""
        if self.output_key is None:
            if len(outputs) != 1:
                raise ValueError(f"One output key expected, got {outputs.keys()}")
            return list(outputs.keys())[0]
        return self.output_key