code: def image_path_from_index(self, index):
          
          assert self.image_set_index is not None, "Dataset not initialized"
          name = self.image_set_index[index]
          image_file = os.path.join(self.data_path, 'JPEGImages', name + self.extension)
          assert os.path.exists(image_file), 'Path does not exist: {}'.format(image_file)
          return image_file
docs: given image index, find out full path
  
          Parameters:
          ----------
          index: int
              index of a specific image
          Returns:
          ----------
          full path of this image