given index, find out sub-db and sub-index

        Parameters
        ----------
        index : int
            index of a specific image

        Returns
        ----------
        a tuple (sub-db, sub-index)

def _locate_index(self, index):
        """
        given index, find out sub-db and sub-index

        Parameters
        ----------
        index : int
            index of a specific image

        Returns
        ----------
        a tuple (sub-db, sub-index)
        """
        assert index >= 0 and index < self.num_images, "index out of range"
        pos = self.image_set_index[index]
        for k, v in enumerate(self.imdbs):
            if pos >= v.num_images:
                pos -= v.num_images
            else:
                return (k, pos)