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 __init__(self, obs_dim, act_dim):
      super(Model, self).__init__()
      hid1_size = 256
      hid2_size = 64
  
      self.fc1 = nn.Linear(obs_dim, hid1_size)
      self.fc2 = nn.Linear(hid1_size, hid2_size)
      self.fc3 = nn.Linear(hid2_size, act_dim)
  
file_path: PARL/examples/tutorials/parl2_dygraph/lesson4/homework/policy_gradient_pong/model.py
imports: import paddle
  import paddle.nn
  import paddle.nn.functional
  import parl
label_desc: Use image node_id 1 to create a new Model object from inherited base classes: parl with example: obj = Model(obs_dim, act_dim)