Returns bounding-box detection results by thresholding on scores and
        applying non-maximum suppression (NMS).

def filter_results(self, boxlist, num_classes):
        """Returns bounding-box detection results by thresholding on scores and
        applying non-maximum suppression (NMS).
        """
        # unwrap the boxlist to avoid additional overhead.
        # if we had multi-class NMS, we could perform this directly on the boxlist
        boxes = boxlist.bbox.reshape(-1, num_classes * 4)
        scores = boxlist.get_field("scores").reshape(-1, num_classes)

        device = scores.device
        result = []
        # Apply threshold on detection probabilities and apply NMS
        # Skip j = 0, because it's the background class
        inds_all = scores > self.score_thresh
        for j in range(1, num_classes):
            inds = inds_all[:, j].nonzero().squeeze(1)
            scores_j = scores[inds, j]
            boxes_j = boxes[inds, j * 4 : (j + 1) * 4]
            boxlist_for_class = BoxList(boxes_j, boxlist.size, mode="xyxy")
            boxlist_for_class.add_field("scores", scores_j)
            boxlist_for_class = boxlist_nms(
                boxlist_for_class, self.nms
            )
            num_labels = len(boxlist_for_class)
            boxlist_for_class.add_field(
                "labels", torch.full((num_labels,), j, dtype=torch.int64, device=device)
            )
            result.append(boxlist_for_class)

        result = cat_boxlist(result)
        number_of_detections = len(result)

        # Limit to max_per_image detections **over all classes**
        if number_of_detections > self.detections_per_img > 0:
            cls_scores = result.get_field("scores")
            image_thresh, _ = torch.kthvalue(
                cls_scores.cpu(), number_of_detections - self.detections_per_img + 1
            )
            keep = cls_scores >= image_thresh.item()
            keep = torch.nonzero(keep).squeeze(1)
            result = result[keep]
        return result