Remove images without usable rois

def filter_roidb(self):
        """Remove images without usable rois"""
        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))