code: def get_per_channel_mean(self, names=('train', 'test')):
          
          mean = self.get_per_pixel_mean(names)
          return np.mean(mean, axis=(0, 1))
docs: Args:
              names (tuple[str]): the names ('train' or 'test') of the datasets
  
          Returns:
              An array of three values as mean of each channel, for all images in the given datasets.