docstr: soft update the target net with Polyak averaging 
docstr_tok: ["soft","update","the","target","net","with","Polyak","averaging"]
code: def target_soft_update(self, net, target_net, soft_tau):
      """soft update the target net with Polyak averaging"""
      for target_param, param in zip(
          target_net.trainable_weights, net.trainable_weights
      ):
          target_param.assign(  # copy weight value into target parameters
              target_param * (1.0 - soft_tau) + param * soft_tau
          )
      return target_net
  
imports: import argparse
  import os
  import random
  import time
  import gym
  import matplotlib.pyplot
  import numpy
  import tensorflow
  import tensorflow_probability
  import tensorlayer
  from tensorlayer.layers import Dense
  from tensorlayer.models import Model
label_desc: Use image node_id 3 for calling the SAC obj's underlying member method code with example usage: obj.target_soft_update(net, target_net, soft_tau) and returns: target_net