def jaccard(box_a, box_b):
    """计算两组矩形两两之间的iou
    Args:
        box_a: (tensor) bounding boxes, Shape: [A, 4].
        box_b: (tensor) bounding boxes, Shape: [B, 4].
    Return:
        ious: (tensor) Shape: [A, B]
    """
    inter = intersect(box_a, box_b)
    area_a = ((box_a[:, 2]-box_a[:, 0]) *
              (box_a[:, 3]-box_a[:, 1])).unsqueeze(1).expand_as(inter)  # [A, B]
    area_b = ((box_b[:, 2]-box_b[:, 0]) *
              (box_b[:, 3]-box_b[:, 1])).unsqueeze(0).expand_as(inter)  # [A, B]
    union = area_a + area_b - inter
    return inter / union  # [A, B]
计算两组矩形两两之间的iou
Args:
box_a: (tensor) bounding boxes, Shape: [A, 4].
box_b: (tensor) bounding boxes, Shape: [B, 4].
Return:
ious: (tensor) Shape: [A, B]
ptstructure/layout/ptppyolov2/pt_utils.py