Fixing issue #1530 by starting ann_id to 1 in convert_to_coco_api (#1531)

Author: fsavard-eai

references/detection/coco_utils.py

@@ -145,7 +145,8 @@ def _has_valid_annotation(anno):
 
 def convert_to_coco_api(ds):
     coco_ds = COCO()
-    ann_id = 0
+    # annotation IDs need to start at 1, not 0, see torchvision issue #1530
+    ann_id = 1
     dataset = {'images': [], 'categories': [], 'annotations': []}
     categories = set()
     for img_idx in range(len(ds)):