code: def optimizer(self):
      lr = tf.get_variable(
          "learning_rate", initializer=1e-4, trainable=False
      )
      opt = tf.train.AdamOptimizer(lr, beta1=0.5, beta2=0.9)
      return opt
  
file_path: tensorpack/examples/GAN/BEGAN.py
imports: 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.gpu import get_num_gpu
  import DCGAN
  from GAN import GANModelDesc, GANTrainer
label_desc: Use image node_id 5 for calling the Model obj's underlying member method code with example usage: obj.optimizer() and returns: opt