Add Submanifold Sparse Convolution

[digital-idiot]

It would be very helpful if Submanifold Sparse Convolution can be added to the API.
The paper: https://arxiv.org/pdf/1706.01307.pdf

Similarly: Voxel-MLP from https://www.ecva.net/papers/eccv_2020/papers_ECCV/papers/123690630.pdf