code: def optimizer(self):
      lr = tf.get_variable(
          "learning_rate", initializer=0.01, trainable=False
      )
      opt = tf.train.MomentumOptimizer(lr, 0.9)
      return opt
  
file_path: tensorpack/examples/ResNet/cifar10-resnet.py
imports: import argparse
  import os
  from tensorpack import tfv1
  from tensorpack import *
  from tensorpack.dataflow import dataset
  from tensorpack.tfutils.summary import add_moving_summary, add_param_summary
  from tensorpack.utils.gpu import get_num_gpu
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