Remove empty boxes before NMS (#1019)

Author: fmassa

torchvision/models/detection/roi_heads.py

@@ -501,6 +501,10 @@ def postprocess_detections(self, class_logits, box_regression, proposals, image_
             inds = torch.nonzero(scores > self.score_thresh).squeeze(1)
             boxes, scores, labels = boxes[inds], scores[inds], labels[inds]
 
+            # remove empty boxes
+            keep = box_ops.remove_small_boxes(boxes, min_size=1e-2)
+            boxes, scores, labels = boxes[keep], scores[keep], labels[keep]
+
             # non-maximum suppression, independently done per class
             keep = box_ops.batched_nms(boxes, scores, labels, self.nms_thresh)
             # keep only topk scoring predictions