Gatys et al. CVPR 2017
    ref: Image Style Transfer Using Convolutional Neural Networks

def optimize(args):
    """    Gatys et al. CVPR 2017
    ref: Image Style Transfer Using Convolutional Neural Networks
    """
    if args.cuda:
        ctx = mx.gpu(0)
    else:
        ctx = mx.cpu(0)
    # load the content and style target
    content_image = utils.tensor_load_rgbimage(args.content_image,ctx, size=args.content_size, keep_asp=True)
    content_image = utils.subtract_imagenet_mean_preprocess_batch(content_image)
    style_image = utils.tensor_load_rgbimage(args.style_image, ctx, size=args.style_size)
    style_image = utils.subtract_imagenet_mean_preprocess_batch(style_image)
    # load the pre-trained vgg-16 and extract features
    vgg = net.Vgg16()
    utils.init_vgg_params(vgg, 'models', ctx=ctx)
    # content feature
    f_xc_c = vgg(content_image)[1]
    # style feature
    features_style = vgg(style_image)
    gram_style = [net.gram_matrix(y) for y in features_style]
    # output
    output = Parameter('output', shape=content_image.shape)
    output.initialize(ctx=ctx)
    output.set_data(content_image)
    # optimizer
    trainer = gluon.Trainer([output], 'adam',
                            {'learning_rate': args.lr})
    mse_loss = gluon.loss.L2Loss()

    # optimizing the images
    for e in range(args.iters):
        utils.imagenet_clamp_batch(output.data(), 0, 255)
        # fix BN for pre-trained vgg
        with autograd.record():
            features_y = vgg(output.data())
            content_loss = 2 * args.content_weight * mse_loss(features_y[1], f_xc_c)
            style_loss = 0.
            for m in range(len(features_y)):
                gram_y = net.gram_matrix(features_y[m])
                gram_s = gram_style[m]
                style_loss = style_loss + 2 * args.style_weight * mse_loss(gram_y, gram_s)
            total_loss = content_loss + style_loss
            total_loss.backward()

        trainer.step(1)
        if (e + 1) % args.log_interval == 0:
            print('loss:{:.2f}'.format(total_loss.asnumpy()[0]))

    # save the image
    output = utils.add_imagenet_mean_batch(output.data())
    utils.tensor_save_bgrimage(output[0], args.output_image, args.cuda)