def __call__(self, max_lengths, min_lengths, num_buckets):
raise NotImplementedError<EOL>
Generate bucket keys based on the lengths of sequences and number of buckets.

        Parameters
        ----------
        max_lengths : int of list of int
            Maximum of lengths of sequences.
        min_lengths : int of list of int
            Minimum of lengths of sequences.
        num_buckets : int
            Number of buckets

        Returns
        -------
        bucket_keys : list of int
            A list including the keys of the buckets.