Calculates the overlap between proposal and ground truth boxes.

Some boxes or gt_boxes may have been padded. The returned iou 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 might have paddings with a negative value.

iou a tensor with as a shape of [batch_size, N, MAX_NUM_INSTANCES].