IoU and Generalised IoU for Boxes

Boxes are [x1, y1, x2, y2] with x1 < x2, y1 < y2.

• IoU = intersection area / union area
• GIoU = $\text{IoU} - \frac{|C| - |A \cup B|}{|C|}$, where $C$ is the smallest box enclosing both. Unlike IoU, it still gives a signal (negative values) when boxes don't overlap.

Implement iou(a, b) and giou(a, b) for arrays of boxes of shape (N, 4) paired row by row, returning arrays of shape (N,).