[BUG] boxes_iou_bev returns a large number when boxes has negative edges.

[Qiza-lyhm]

https://github.com/open-mmlab/OpenPCDet/blob/dd2edf9da0944238d3711f8145cc5b86c65f5a79/pcdet/ops/iou3d_nms/src/iou3d_nms_kernel.cu#L233

当A、B两个Box中存在负数边长时，其面积Sa和Sb可能被计算为负值。 因此此处的Sa+Sb-S_overlap结果可能为负值，并导致s_overlap / fmaxf(sa + sb - s_overlap, EPS) 为一个无意义的大数。

是否应该要确保sa、sb为正数，对其做abs处理？或与MMCV中的类似计算一样直接返回IoU=0？ 参考MMCV的实现：https://github.com/open-mmlab/mmcv/blob/b9bdfeb6f6aa67372d18b2ed5a10de28fc179d11/mmcv/ops/csrc/common/box_iou_rotated_utils.hpp#L334

[sshaoshuai]

Thank you very much, that's a good point, maybe we can add an assert function in the python iou3d calculation codes. So the users do not need to re-compile the CUDA codes.

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