given image index, find out annotation path

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

        Returns:
        ----------
        full path of annotation file

def _label_path_from_index(self, index):
        """
        given image index, find out annotation path

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

        Returns:
        ----------
        full path of annotation file
        """
        label_file = os.path.join(self.data_path, 'Annotations', index + '.xml')
        assert os.path.exists(label_file), 'Path does not exist: {}'.format(label_file)
        return label_file