Start a new episode.

def reset(self):
    """Start a new episode."""
    self._episode_steps = 0
    if self._episode_count:
      # No need to restart for the first episode.
      self._restart()

    self._episode_count += 1
    logging.info("Starting episode: %s", self._episode_count)
    self._metrics.increment_episode()

    self._last_score = [0] * self._num_agents
    self._state = environment.StepType.FIRST
    if self._realtime:
      self._last_step_time = time.time()
      self._target_step = 0

    return self._observe()