code: def _decay():
      """L2 weight decay loss."""
      costs = []
      for var in tf.trainable_variables():
          if var.op.name.find("DW") > 0:
              costs.append(tf.nn.l2_loss(var))
      return tf.add_n(costs)
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import numpy
  import tensorflow
  from cleverhans.serial import NoRefModel
label_desc: Use image node_id 4 for calling a global function with example usage: _decay() and returns: tf