code: def transform(im, mean, std):
      
      im_tensor = np.zeros((3, im.shape[0], im.shape[1]))
      for i in range(3):
          im_tensor[i, :, :] = (im[:, :, 2 - i] - mean[i]) / std[i]
      return im_tensor
docs: transform into mxnet tensor,
      subtract pixel size and transform to correct format
      :param im: [height, width, channel] in BGR
      :param mean: [RGB pixel mean]
      :param std: [RGB pixel std var]
      :return: [batch, channel, height, width]