code: def _label_path_from_index(self, index):
          
          label_file = os.path.join(self.label_dir, index + self.label_extension)
          assert os.path.exists(label_file), 'Path does not exist: {}'.format(label_file)
          return label_file
docs: given image index, find out annotation path
  
          Parameters:
          ----------
          index: int
              index of a specific image
  
          Returns:
          ----------
          full path of annotation file