code: def optimizer(self):
      lr = tf.get_variable(
          "learning_rate", initializer=1e-4, trainable=False
      )
      opt = tf.train.AdamOptimizer(lr)
      return opt
  
imports: import argparse
  import numpy
  import os
  import cv2
  import six
  import tensorflow
  from tensorpack import *
  from tensorpack.tfutils.scope_utils import auto_reuse_variable_scope
  from tensorpack.tfutils.summary import add_moving_summary
  from tensorpack.utils import logger
  from tensorpack.utils.gpu import get_num_gpu
  from data_sampler import CenterSquareResize, ImageDataFromZIPFile, ImageDecode, RejectTooSmallImages
  from GAN import GANModelDesc, SeparateGANTrainer
label_desc: Use image node_id 4 for calling the Model obj's underlying member method code with example usage: obj.optimizer() and returns: opt