unbatch image_id in CocoDetection wrapper (#7232)

torchvision/prototype/datapoints/_dataset_wrapper.py

@@ -206,6 +206,10 @@ def coco_dectection_wrapper_factory(dataset):
 
         batched_target = list_of_dicts_to_dict_of_lists(target)
 
+        image_ids = batched_target.pop("image_id")
+        image_id = batched_target["image_id"] = image_ids.pop()
+        assert all(other_image_id == image_id for other_image_id in image_ids)
+
         spatial_size = tuple(F.get_spatial_size(image))
         batched_target["boxes"] = datapoints.BoundingBox(
             batched_target["bbox"],