Make Conv2D support 4 dims padding

Questions
1

We have discussed many things about extending conv2d padding to 4 dims padding. (top, bottom, left, right) like this issue: https://github.com/dmlc/nnvm/pull/472

We have done this support completely and run models very well (TF MobilenetV1 / Mobilenet V2 / Deeplab V3 / Resnet50 and so on). We want to contribute back to community. However, this affects many places, we want to make sure community want to accept this change or not. Then we can pull request it.

CC: @tqchen @Huyuwei @srkreddy1238

2

@FrozenGene you may describe the changes or share PR to get an understanding of the changes.

3

We will collect the materials and describe it more detail soon. Thanks very much!