missing pooling operators

[CarloLucibello]

• [ ] DiffPool. paper: https://arxiv.org/abs/1806.08804
• [ ] MinCutPool. paper: https://arxiv.org/abs/1907.00481
• [ ] Graculus. paper: https://www.cs.utexas.edu/users/inderjit/public_papers/multilevel_pami.pdf / https://arxiv.org/pdf/1606.09375.pdf