code: def _actor_learn(self, obs):
      action = self.model.policy(obs)
      Q = self.model.value(obs, action)
      cost = layers.reduce_mean(-1.0 * Q)
      optimizer = fluid.optimizer.AdamOptimizer(self.actor_lr)
      optimizer.minimize(
          cost, parameter_list=self.model.get_actor_params()
      )
      return cost
  
file_path: PARL/parl/algorithms/fluid/ddpg.py
imports: import warnings
  from parl.core.fluid import layers
  from copy import deepcopy
  from paddle import fluid
  from parl.core.fluid.algorithm import Algorithm
label_desc: Use image node_id 4 for calling the DDPG obj's underlying member method code with example usage: obj._actor_learn(obs) and returns: cost