Hepler function to write image list into the file.
    The format is as below,
    integer_image_index \t float_label_index \t path_to_image
    Note that the blank between number and tab is only used for readability.
    Parameters
    ----------
    path_out: string
    image_list: list

def write_list(path_out, image_list):
    """Hepler function to write image list into the file.
    The format is as below,
    integer_image_index \t float_label_index \t path_to_image
    Note that the blank between number and tab is only used for readability.
    Parameters
    ----------
    path_out: string
    image_list: list
    """
    with open(path_out, 'w') as fout:
        for i, item in enumerate(image_list):
            line = '%d\t' % item[0]
            for j in item[2:]:
                line += '%f\t' % j
            line += '%s\n' % item[1]
            fout.write(line)