transform from mxnet im_tensor to ordinary RGB image
    im_tensor is limited to one image
    :param im_tensor: [batch, channel, height, width]
    :param mean: [RGB pixel mean]
    :param std: [RGB pixel std var]
    :return: im [height, width, channel(RGB)]

def transform_inverse(im_tensor, mean, std):
    """
    transform from mxnet im_tensor to ordinary RGB image
    im_tensor is limited to one image
    :param im_tensor: [batch, channel, height, width]
    :param mean: [RGB pixel mean]
    :param std: [RGB pixel std var]
    :return: im [height, width, channel(RGB)]
    """
    assert im_tensor.shape[0] == 3
    im = im_tensor.transpose((1, 2, 0))
    im = im * std + mean
    im = im.astype(np.uint8)
    return im