code: def filter_roidb(self):
          
          num_roidb = len(self._roidb)
          self._roidb = [roi_rec for roi_rec in self._roidb if len(roi_rec['gt_classes'])]
          num_after = len(self._roidb)
          logger.info('filter roidb: {} -> {}'.format(num_roidb, num_after))
docs: Remove images without usable rois