code: def optimizer(self):
      lr = tf.get_variable(
          "learning_rate", initializer=1.0, trainable=False
      )
      opt = tf.train.GradientDescentOptimizer(lr)
      return optimizer.apply_grad_processors(
          opt, [gradproc.GlobalNormClip(5)]
      )
  
file_path: tensorpack/examples/PennTreebank/PTB-LSTM.py
imports: import argparse
  import numpy
  import os
  import tensorflow
  from tensorpack import *
  from tensorpack.tfutils import gradproc, optimizer, summary
  from tensorpack.utils import logger
  from tensorpack.utils.argtools import memoized_ignoreargs
  from tensorpack.utils.fs import download, get_dataset_path
  import reader
  from reader import ptb_producer
label_desc: Use image node_id 4 for calling the Model obj's underlying member method code with example usage: obj.optimizer() and returns: optimizer