box helpers (box_convert, box_iou)

Also written: box_convert, box_iou, torchvision.ops.box_convert, torchvision.ops.box_iou

`torchvision.ops.box_convert` converts boxes between formats such as XYXY and XYWH. `box_iou` computes the IoU of every box in its first argument against every box in its second, one row per true box and one column per guess.