langchain/chains/base.py/Chain/_acall
class Chain:    async def _acall(
        self,
        inputs: Dict[str, Any],
        run_manager: Optional[AsyncCallbackManagerForChainRun] = None,
    ) -> Dict[str, Any]:
        """Run the logic of this chain and return the output."""
        raise NotImplementedError("Async call not supported for this chain type.")