code: def peak_signal_to_noise_ratio(true, pred):
      """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)
      """
      return (
          10.0
          * tf.log(1.0 / mean_squared_error(true, pred))
          / tf.log(10.0)
      )
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  from six.moves import reduce
  from tensor2tensor.layers import common_layers
  from tensor2tensor.layers import common_video
  from tensor2tensor.models.video import epva_params
  from tensor2tensor.models.video import sv2p
  from tensor2tensor.utils import contrib
  from tensor2tensor.utils import registry
  import tensorflow.compat.v1
  from tensorflow.contrib.framework.python.ops import arg_scope
  from tensorflow.contrib.slim.python.slim.nets import vgg
label_desc: Use image node_id 9 for calling a global function with example usage: peak_signal_to_noise_ratio(true, pred) and returns: unknown