def _build_single_dataset(
        self,
        split: Union[str, ReadInstruction, Split],
        run_post_process: bool,
        verification_mode: VerificationMode,
        in_memory: bool = False,
    ):
        """as_dataset for a single split."""
        if not isinstance(split, ReadInstruction):
            split = str(split)
            if split == "all":
                split = "+".join(self.info.splits.keys())
            split = Split(split)