code: def peak_signal_to_noise_ratio(true, pred):
    
    return 10.0 * tf.log(1.0 / mean_squared_error(true, pred)) / tf.log(10.0)
docs: Image quality metric based on maximal signal power vs. power of the noise.
  
    Args:
      true: the ground truth image.
      pred: the predicted image.
    Returns:
      peak signal to noise ratio (PSNR)