A value net function.

def value_net(rng_key,
              batch_observations_shape,
              num_actions,
              bottom_layers=None):
  """A value net function."""
  del num_actions

  if bottom_layers is None:
    bottom_layers = []
  bottom_layers.extend([
      layers.Dense(1),
  ])
  net = layers.Serial(*bottom_layers)
  return net.initialize(batch_observations_shape, rng_key), net