code: def value(self, obs, act):
      x = self.fc1(obs)
      concat = layers.concat([x, act], axis=1)
      x = self.fc2(concat)
      Q = self.fc3(x)
      Q = layers.squeeze(Q, axes=[1])
      return Q
  
file_path: PARL/examples/tutorials/homework/lesson5/ddpg_quadrotor/quadrotor_model.py
imports: import paddle.fluid
  import parl
  from parl import layers
label_desc: Use image node_id 2 for calling the CriticModel obj's underlying member method code with example usage: obj.value(obs, act) and returns: Q