unbatch image_id in CocoDetection wrapper (#7232)

Author: pmeier

torchvision/prototype/datapoints/_dataset_wrapper.py

@@ -206,6 +206,10 @@ def wrapper(sample):
 
  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"],