Add single zero row/column to observation if needed.

def observation(self, frame):
    """Add single zero row/column to observation if needed."""
    if frame.shape == self.observation_space.shape:
      return frame
    else:
      extended_frame = np.zeros(self.observation_space.shape,
                                self.observation_space.dtype)
      assert self.HW_AXES == (0, 1)
      extended_frame[:frame.shape[0], :frame.shape[1]] = frame
      return extended_frame