'Returns
dataset: dict of shape {user_id:[item_id,item_id2,...]}
Load the dataset which the input can be the
{user_id:{item_id:preference,...},...}
or the {user_id:[item_id,item_id2,...],...}'

def _load_dataset(self, dataset):

    if dataset:
        key = dataset.keys()[0]
        if isinstance(dataset[key], dict):
            for key in dataset:
                dataset[key] = dataset[key].keys()
    return dataset
