Returns a tuple of sum of raw and processed rewards.

def reward(self):
    """Returns a tuple of sum of raw and processed rewards."""
    raw_rewards, processed_rewards = 0, 0
    for ts in self.time_steps:
      # NOTE: raw_reward and processed_reward are None for the first time-step.
      if ts.raw_reward is not None:
        raw_rewards += ts.raw_reward
      if ts.processed_reward is not None:
        processed_rewards += ts.processed_reward
    return raw_rewards, processed_rewards