given image index, find out full path

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

        Returns
        ----------
        full path of this image

def image_path_from_index(self, index):
        """
        given image index, find out full path

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

        Returns
        ----------
        full path of this image
        """
        assert self.image_set_index is not None, "Dataset not initialized"
        pos = self.image_set_index[index]
        n_db, n_index = self._locate_index(index)
        return self.imdbs[n_db].image_path_from_index(n_index)