code: def _load_image_set_index(self, shuffle):
          
          assert os.path.exists(self.list_file), 'Path does not exists: {}'.format(self.list_file)
          with open(self.list_file, 'r') as f:
              image_set_index = [x.strip() for x in f.readlines()]
          if shuffle:
              np.random.shuffle(image_set_index)
          return image_set_index
docs: find out which indexes correspond to given image set (train or val)
  
          Parameters:
          ----------
          shuffle : boolean
              whether to shuffle the image list
          Returns:
          ----------
          entire list of images specified in the setting