Get the index name with the data for a metric class
        :param metric_cls: a metric class
        :return: the name of the index with the data for the metric

def get_metric_index(self, metric_cls):
        """
        Get the index name with the data for a metric class
        :param metric_cls: a metric class
        :return: the name of the index with the data for the metric
        """
        ds = self.class2ds[metric_cls.ds]
        if self.index_dict[ds]:
            index_name = self.index_dict[ds]
        else:
            index_name = self.ds2index[metric_cls.ds]
        return index_name