code: def _locate_index(self, index):
          
          assert index >= 0 and index < self.num_images, "index out of range"
          pos = self.image_set_index[index]
          for k, v in enumerate(self.imdbs):
              if pos >= v.num_images:
                  pos -= v.num_images
              else:
                  return (k, pos)
docs: given index, find out sub-db and sub-index
  
          Parameters
          ----------
          index : int
              index of a specific image
  
          Returns
          ----------
          a tuple (sub-db, sub-index)