code: def actor_learn(self, obs):
      action, log_pi = self.sample(obs)
      qf1_pi, qf2_pi = self.critic.value(obs, action)
      min_qf_pi = layers.elementwise_min(qf1_pi, qf2_pi)
      cost = log_pi * self.alpha - min_qf_pi
      cost = layers.reduce_mean(cost)
      optimizer = fluid.optimizer.AdamOptimizer(self.actor_lr)
      optimizer.minimize(cost, parameter_list=self.actor.parameters())
  
      return cost
  
file_path: PARL/parl/algorithms/fluid/sac.py
imports: from parl.core.fluid import layers
  from copy import deepcopy
  import numpy
  from paddle import fluid
  from paddle.fluid.layers import Normal
  from parl.core.fluid.algorithm import Algorithm
label_desc: Use image node_id 5 for calling the SAC obj's underlying member method code with example usage: obj.actor_learn(obs) and returns: cost