Deformable convolution

[MaratKhabibullin]

Please support Deformable convolution layer.

[FedyuninV]

That will require some time to implement (PS ROI pooling + deformConv for all the platforms, optimizations, etc.), that's why it's not gonna be done in near future (sadly).

Feel free to implement it (if you have time and will), we will be able to review it.