More explicit bounding box formats

### Feature request I am having a difficult time managing different bounding box formats in this library. For example, the DETR model preprocessor input format is COCO (xywh), but the output of the postprocessor is a pascal_voc (xyxy). Why not keep everything the same format? Furthermore, the documentation does not say which format is which so it's required to dig through the source to identify that information. ### Motivation Ease of use ### Your contribution Just a request