code: def _actor_learn(self, obs):
      # Compute actor loss and Update the frozen target models
      actor_loss = -self.model.value(obs, self.model.policy(obs)).mean()
  
      # Optimize the actor
      self.actor_optimizer.clear_grad()
      actor_loss.backward()
      self.actor_optimizer.step()
      return actor_loss
  
file_path: PARL/parl/algorithms/paddle/ddpg.py
imports: import parl
  import paddle
  import paddle.nn.functional
  from copy import deepcopy
  from parl.utils.utils import check_model_method
label_desc: Use image node_id 5 for calling the DDPG obj's underlying member method code with example usage: obj._actor_learn(obs) and returns: actor_loss