Add Deformable ConvNets

[souravsingh]

The codebase is taken from https://github.com/yuyu2172/deformable-conv

[aonotas]

I will review this PR later!
Thank you for this PR.
Please wait a little.

[aonotas]

Could you add README.md in ./deformable_convnets/README.md?

I think all file path can be in ./deformable_convnets/. In my opinion, mnist directory is not needed.

Please write requirements library in README.

$ pip install chainercv

[aonotas]

I am trying to run this PR code,
Is it fine for you?

$ python deformable_convnets/mnist/train_mnist.py --gpu 0
GPU: 0
# Minibatch-size: 100
# epoch: 20
deformable: 1

epoch       main/loss   validation/main/loss  main/accuracy  validation/main/accuracy  elapsed_time
1           2.30162     2.30107               0.112067       0.1135                    95.9675
2           2.30129     2.30102               0.112367       0.1135                    116.356
3           2.30125     2.30106               0.112367       0.1135                    135.722
4           2.30127     2.30101               0.112367       0.1135                    154.666
5           2.30129     2.30095               0.112367       0.1135                    173.193
6           2.30129     2.30103               0.112367       0.1135                    192.252
7           2.30128     2.3011                0.112367       0.1135                    211.127
8           2.30128     2.30107               0.112367       0.1135                    229.825
9           2.30126     2.30098               0.112367       0.1135                    250.089
10          2.30127     2.30106               0.112367       0.1135                    269.097

If you could, please add example result of this code in README.md?

[souravsingh]

@aonotas The result looks good. Should we also add a model for Pascal VOC object detection, since it is present in the repo?

[aonotas]

If you could add a model for Pascal VOC object detection and its results, please add it!
Thank you for your great work!