Support empty predictions when visualizing results

Summary:
This PR fixes a potential error when no predictions found in `tools/visualize_json_results.py`.
Pull Request resolved: facebookresearch#1166

Reviewed By: rbgirshick

Differential Revision: D20888849

Pulled By: ppwwyyxx

fbshipit-source-id: a53d67a7f3ee63070defedb6ca3812d0ae5d6cc9

tools/visualize_json_results.py

@@ -22,7 +22,7 @@ def create_instances(predictions, image_size):
     score = np.asarray([x["score"] for x in predictions])
     chosen = (score > args.conf_threshold).nonzero()[0]
     score = score[chosen]
-    bbox = np.asarray([predictions[i]["bbox"] for i in chosen])
+    bbox = np.asarray([predictions[i]["bbox"] for i in chosen]).reshape(-1, 4)
     bbox = BoxMode.convert(bbox, BoxMode.XYWH_ABS, BoxMode.XYXY_ABS)
 
     labels = np.asarray([dataset_id_map(predictions[i]["category_id"]) for i in chosen])