code: def _get_ob(self):
      assert len(self.frames) == self.k
      if self.obs_format == "NHWC":
          return np.stack(self.frames, axis=2)
      else:
          return np.array(self.frames)
  
imports: import time
  import numpy
  from collections import deque
  import gym
  from gym import spaces
  from parl.env.compat_wrappers import CompatWrapper, is_gym_version_ge, V_NPRANDOM_CHANGED
  import cv2
label_desc: Use image node_id 4 for calling the FrameStack obj's underlying member method code with example usage: obj._get_ob() and returns: np, np