img: an RGB image of shape (s, 2s, 3).
    :return: [input, output]

def split_input(img):
    """
    img: an RGB image of shape (s, 2s, 3).
    :return: [input, output]
    """
    # split the image into left + right pairs
    s = img.shape[0]
    assert img.shape[1] == 2 * s
    input, output = img[:, :s, :], img[:, s:, :]
    if args.mode == 'BtoA':
        input, output = output, input
    if IN_CH == 1:
        input = cv2.cvtColor(input, cv2.COLOR_RGB2GRAY)[:, :, np.newaxis]
    if OUT_CH == 1:
        output = cv2.cvtColor(output, cv2.COLOR_RGB2GRAY)[:, :, np.newaxis]
    return [input, output]