def _check_input_labels(self, labels: List[int]) -> None:
        """
        The input must satisfy the conditions described in
        the class documentation.

        Args:
            labels: labels of the samples in the batch
        """
        labels_counter = Counter(labels)
        assert all(n > 1 for n in labels_counter.values())
        assert len(labels_counter) > 1

        The input must satisfy the conditions described in
        the class documentation.

        Args:
            labels: labels of the samples in the batch
        
catalyst/contrib/data/sampler_inbatch.py