[Feature Request] Add support for pooling blocks

[Tortoise0Knight]

e.g. The implementation in MinkowskiEngine: https://nvidia.github.io/MinkowskiEngine/pooling.html#minkowskimaxpooling.
I only found global_max_pool()

[ys-2020]

Hi. We haven't implemented those pooling kernels yet. We will consider to implement them. Thank you for reaching out!

[YilmazKadir]

I also need average pooling for my application case and would appreciate it if you could implement this. Also, I would be happy if you could suggest a way to implement average pooling with convolutions. I thought using convolutions with all kernel elements = 1/N but N needs to be the number of active voxels inside the receptive field and I do not know how I can get that number.

[kabouzeid]

Yes, that would be extremely useful. I was in the process of migrating my code for Minkowski, but sadly the lack of pooling layers make this impossible now.

@zhijian-liu @kentang-mit