Add check for empty bboxes

Summary:
Pull Request resolved: https://github.com/facebookresearch/d2go/pull/616

The check for valid bboxes doesn't verify that the bbox list has exactly 4 elements, and crashes the training instead of marking empty bboxes as invalid (see f472700454).

Reviewed By: tglik

Differential Revision: D48653084

fbshipit-source-id: 2d47fb267c5e51ab27798662ae739014f3d310e4

d2go/data/extended_coco.py

@@ -142,6 +142,7 @@ def convert_coco_text_to_coco_detection_json(
 def valid_bbox(bbox_xywh: List[int], img_w: int, img_h: int) -> bool:
     if (
         bbox_xywh is None
+        or not len(bbox_xywh) == 4
         or (bbox_xywh[3] == 0 or bbox_xywh[2] == 0)
         or not (0 <= bbox_xywh[0] <= img_w - bbox_xywh[2])
         or not (0 <= bbox_xywh[1] <= img_h - bbox_xywh[3])