Repeat action, sum reward, and max over last observations.

def step(self, action):
    """Repeat action, sum reward, and max over last observations."""
    total_reward = 0.0
    done = None
    for i in range(self._skip):
      obs, reward, done, info = self.env.step(action)
      if i == self._skip - 2:
        self._obs_buffer[0] = obs
      if i == self._skip - 1:
        self._obs_buffer[1] = obs
      total_reward += reward
      if done:
        break
    # Note that the observation on the done=True frame doesn't matter.
    max_frame = self._obs_buffer.max(axis=0)
    return max_frame, total_reward, done, info