def model_fn(env):
    x = Placeholder(tf.float32, env.observation_space.shape, 'x')
    return Policy('pi', x, ac_space=env.action_space)
def model_fn(env):
    x = Placeholder(tf.float32, env.observation_space.shape, 'x')
    return Policy('pi', x, ac_space=env.action_space)