Takes images scaled to [0, 1] and returns them appropriately scaled and
    mean-subtracted for VGG-16

def _vgg16_data_prep(batch):
    """
    Takes images scaled to [0, 1] and returns them appropriately scaled and
    mean-subtracted for VGG-16
    """
    from mxnet import nd
    mean = nd.array([123.68, 116.779, 103.939], ctx=batch.context)
    return nd.broadcast_sub(255 * batch, mean.reshape((-1, 1, 1)))