Add deformable convolution DCNv2 support

[breznak]

Recently there was a great PR 👍 adding deformable convolution support DCN (v1) support to this repo. Meanwhile improved version DCNv2 appeared, so I'd like to support that.

• The original DCN PR could be used as a referemce Add Deformable Convolution operation. #1586 Add Deformable Convolution operation. #1586 (comment)
• port code from mmdetection or Add Deformable Convolution operation. #1586 (comment)
  • which one would be better?

[ksnzh]

Mmdetection has two implementations: 1) deform_conv 2) modulated_deform_conv. The difference between those two is that modulated_deform_conv one more argument mask. Also, they have different implementations in cpu and cuda, and he torchvision's version is the former.

class DeformConvFunction(Function):

    @staticmethod
    def forward(ctx,
                input,
                offset,
                weight,
                stride=1,
                padding=0,
                dilation=1,
                groups=1,
                deformable_groups=1,
                im2col_step=64):

class ModulatedDeformConvFunction(Function):

    @staticmethod
    def forward(ctx,
                input,
                offset,
                mask,
                weight,
                bias=None,
                stride=1,
                padding=0,
                dilation=1,
                groups=1,
                deformable_groups=1):

[inspiros]

In the mean time, my Covid work might help someone https://github.com/inspiros/tvdcn