Calculates the GIOU between proposal and ground truth boxes.

The generalized intersection of union is an adjustment of the traditional IOU metric which provides continuous updates even for predictions with no overlap. This metric is defined in Note, some gt_boxes may have been padded. The returned giou tensor for these boxes will be -1.

boxes a Tensor with a shape of [batch_size, N, 4]. N is the number of proposals before groundtruth assignment (e.g., rpn_post_nms_topn). The last dimension is the pixel coordinates in [ymin, xmin, ymax, xmax] form.
gt_boxes a Tensor with a shape of [batch_size, max_num_instances, 4]. This tensor may have paddings with a negative value and will also be in the [ymin, xmin, ymax, xmax] format.

giou a Tensor with as a shape of [batch_size, N, max_num_instances].