cascade voxel decoder for occnet-challenge version

[autumn-wong]

Thanks for your great work!
I'm having some problems reading through the occnet-challenge version of the code, I can't find the code for the cascade voxel decoder in that challenge version, please help point out the location of the relevant code.

[sephyli]

We only provide a BEVFormer baseline In the challenge branch. You can find the related code is in the occnet branch.