docs: def crop(self, box):
          """
          Cropss a rectangular region from this bounding box. The box is a
          4-tuple defining the left, upper, right, and lower pixel
          coordinate.
          """
          xmin, ymin, xmax, ymax = self._split_into_xyxy()
          w, h = box[2] - box[0], box[3] - box[1]
          cropped_xmin = (xmin - box[0]).clamp(min=0, max=w)
          cropped_ymin = (ymin - box[1]).clamp(min=0, max=h)
          cropped_xmax = (xmax - box[0]).clamp(min=0, max=w)
          cropped_ymax = (ymax - box[1]).clamp(min=0, max=h)
  
          # TODO should I filter empty boxes here?
          if False:
              is_empty = (cropped_xmin == cropped_xmax) | (cropped_ymin == cropped_ymax)
  
          cropped_box = torch.cat(
              (cropped_xmin, cropped_ymin, cropped_xmax, cropped_ymax), dim=-1
          )
          bbox = BoxList(cropped_box, (w, h), mode="xyxy")
          # bbox._copy_extra_fields(self)
          for k, v in self.extra_fields.items():
              if not isinstance(v, torch.Tensor):
                  v = v.crop(box)
              bbox.add_field(k, v)
          return bbox.convert(self.mode)