Get `item` into a batch. Optionally `detach` and `denorm`.

def one_item(self, item, detach:bool=False, denorm:bool=False, cpu:bool=False):
        "Get `item` into a batch. Optionally `detach` and `denorm`."
        ds = self.single_ds
        with ds.set_item(item):
            return self.one_batch(ds_type=DatasetType.Single, detach=detach, denorm=denorm, cpu=cpu)