Returns appropriate `Dataset` for validation, training, or test (`ds_type`).

def dl(self, ds_type:DatasetType=DatasetType.Valid)->DeviceDataLoader:
        "Returns appropriate `Dataset` for validation, training, or test (`ds_type`)."
        #TODO: refactor
        return (self.train_dl if ds_type == DatasetType.Train else
                self.test_dl if ds_type == DatasetType.Test else
                self.valid_dl if ds_type == DatasetType.Valid else
                self.single_dl if ds_type == DatasetType.Single else
                self.fix_dl)