Free Booster's Datasets.

        Returns
        -------
        self : Booster
            Booster without Datasets.

def free_dataset(self):
        """Free Booster's Datasets.

        Returns
        -------
        self : Booster
            Booster without Datasets.
        """
        self.__dict__.pop('train_set', None)
        self.__dict__.pop('valid_sets', None)
        self.__num_dataset = 0
        return self