Questions on nan

[xyupeng]

Hi there, This is a great work and many thanks for releasing the code. When I look into the code, I find some places where nan is handled, like https://github.com/megvii-research/PETR/blob/7b046e28a03dcb296987eb3e76cbf761f52a37f0/projects/mmdet3d_plugin/models/dense_heads/petr_head.py#L419-L420 and https://github.com/megvii-research/PETR/blob/7b046e28a03dcb296987eb3e76cbf761f52a37f0/projects/mmdet3d_plugin/models/dense_heads/petr_head.py#L625-L626

I wonder in which cases nan would occur? Do you have any advice?

Thanks in advance.