code: def image_path_from_index(self, index):
          
          assert self.image_set_index is not None, "Dataset not initialized"
          pos = self.image_set_index[index]
          n_db, n_index = self._locate_index(index)
          return self.imdbs[n_db].image_path_from_index(n_index)
docs: given image index, find out full path
  
          Parameters
          ----------
          index: int
              index of a specific image
  
          Returns
          ----------
          full path of this image