Adds new frame to (initial) frame stack, removes last one.

def add_to_initial_stack(self, frame):
    """Adds new frame to (initial) frame stack, removes last one."""
    if not self._setable_initial_frames:
      raise ValueError(
          "This instance does not allow to manually set initial frame stack.")
    assert_msg = "{}, {}".format(frame.shape, self._initial_frames.shape[:1])
    assert frame.shape == self._initial_frames.shape[2:], assert_msg
    initial_frames = np.roll(self._initial_frames, shift=-1, axis=1)
    initial_frames[0, -1, ...] = frame
    self._initial_frames = initial_frames