Focalloss can not converge on spconv 2.2/2.3

[yukang2017]

Hi @traveller59 ,

Thanks for your excellent code.

We find that focal loss fails to converge on spconv 2.2/2.3. I have tested on all versions from 2.2.2 to the newest. However, it works on any versions in spconv 2.1.

In addition, other loss, like regression / L1 loss, works well, regardless of spconv versions.

Would you please provide me some hint on this case?

Regards, Yukang Chen