Return a string representing the statistics of the bucketing sampler.

        Returns
        -------
        ret : str
            String representing the statistics of the buckets.

def stats(self):
        """Return a string representing the statistics of the bucketing sampler.

        Returns
        -------
        ret : str
            String representing the statistics of the buckets.
        """
        ret = '{name}:\n' \
            '  sample_num={sample_num}, batch_num={batch_num}\n' \
            '  key={bucket_keys}\n' \
            '  cnt={bucket_counts}\n' \
            '  batch_size={bucket_batch_sizes}'\
            .format(name=self.__class__.__name__,
                    sample_num=len(self._lengths),
                    batch_num=len(self._batch_infos),
                    bucket_keys=self._bucket_keys,
                    bucket_counts=[len(sample_ids) for sample_ids in self._bucket_sample_ids],
                    bucket_batch_sizes=self._bucket_batch_sizes)
        return ret