code: def _load_image_set_index(self, shuffle):
          
          self.num_images = 0
          for db in self.imdbs:
              self.num_images += db.num_images
          indices = list(range(self.num_images))
          if shuffle:
              random.shuffle(indices)
          return indices
docs: get total number of images, init indices
  
          Parameters
          ----------
          shuffle : bool
              whether to shuffle the initial indices