Perform step(action) on environments and update initial_frame_stack.

def _step_envs(self, action):
    """Perform step(action) on environments and update initial_frame_stack."""
    self._frame_counter += 1
    real_env_step_tuple = self.real_env.step(action)
    sim_env_step_tuple = self.sim_env.step(action)
    self.sim_env.add_to_initial_stack(real_env_step_tuple[0])
    return self._pack_step_tuples(real_env_step_tuple, sim_env_step_tuple)