class_docstr: 使用全连接网络.
  
  参数:
      obs_dim (int): 观测空间的维度.
      act_dim (int): 动作空间的维度.
code: def forward(self, obs):
      h1 = F.relu(self.fc1(obs))
      h2 = F.relu(self.fc2(h1))
      Q = self.fc3(h2)
      return Q
  
file_path: PARL/examples/tutorials/parl2_dygraph/lesson3/dqn/model.py
imports: import paddle
  import paddle.nn
  import paddle.nn.functional
  import parl
label_desc: Use image node_id 2 for calling the Model obj's underlying member method code with example usage: obj.forward(obs) and returns: Q