docs: def keep_only_positive_boxes(boxes):
      """
      Given a set of BoxList containing the `labels` field,
      return a set of BoxList for which `labels > 0`.
  
      Arguments:
          boxes (list of BoxList)
      """
      assert isinstance(boxes, (list, tuple))
      assert isinstance(boxes[0], BoxList)
      assert boxes[0].has_field("labels")
      positive_boxes = []
      positive_inds = []
      num_boxes = 0
      for boxes_per_image in boxes:
          labels = boxes_per_image.get_field("labels")
          inds_mask = labels > 0
          inds = inds_mask.nonzero().squeeze(1)
          positive_boxes.append(boxes_per_image[inds])
          positive_inds.append(inds_mask)
      return positive_boxes, positive_inds