code: def optimize(
          self,
          re_encoder_grads_list,
          decoder_grads_list,
          encoder_grads_list,
          learning_rate,
          epoch
      ):
          '''
          Back propagation.
          
          Args:
              re_encoder_grads_list:  re-encoder's `list` of graduations.
              decoder_grads_list:     decoder's `list` of graduations.
              encoder_grads_list:     encoder's `list` of graduations.
              learning_rate:          Learning rate.
              epoch:                  Now epoch.
          '''
          self.__retrospective_encoder.optimize(re_encoder_grads_list, learning_rate, epoch)
          self.__encoder_decoder_controller.optimize(
              decoder_grads_list,
              encoder_grads_list,
              learning_rate,
              epoch
          f optimize(
          self,
          re_encoder_grads_list,
          decoder_grads_list,
          encoder_grads_list,
          learning_rate,
          epoch
      ):
          '''
          Back propagation.
          
          Args:
              re_encoder_grads_list:  re-encoder's `list` of graduations.
              decoder_grads_list:     decoder's `list` of graduations.
              encoder_grads_list:     encoder's `list` of graduations.
              learning_rate:          Learning rate.
              epoch:                  Now epoch.
          '''
          self.__retrospective_encoder.optimize(re_encoder_grads_list, learning_rate, epoch)
          self.__encoder_decoder_controller.optimize(
              decoder_grads_list,
              encoder_grads_list,
              learning_rate,
              epoch
          )
docs: Back propagation.
          
          Args:
              re_encoder_grads_list:  re-encoder's `list` of graduations.
              decoder_grads_list:     decoder's `list` of graduations.
              encoder_grads_list:     encoder's `list` of graduations.
              learning_rate:          Learning rate.
              epoch:                  Now epoch.