code: def _create_optimizer_node(self):
          
          self.lr_var = tf.Variable(0.0, trainable=False)
          tvars = tf.trainable_variables()
          grads, _ = tf.clip_by_global_norm(tf.gradients(self.cost, tvars),
                                            self.max_grad_norm)
          optimizer = tf.train.GradientDescentOptimizer(self.lr_var)
          self._train_op = optimizer.apply_gradients(zip(grads, tvars))
docs: Create the optimizer node of the graph.