Change code for compatibility with pytorch 0.4.1

[songheony]

Since C extension APIs are changed in PyTorch 0.4.1, you can not build FPN and FRCNN with PyTorch 0.4.1.
Thus, I've forked FPN and FRCNN and changed it by following jwyang's faster-rcnn.pytorch for code to be compatible with PyTorch 0.4.1.

[songheony]

I've compiled FPN and FRCNN with gcc++-6 and Cuda toolkit 9.2
Then, install PyTorch by Anaconda (conda install -y pytorch=0.4.1 torchvision cuda92 -c pytorch).

[timmeinhardt]

Thank you for this contribution but this branch is not really maintained anymore. Is there a particular reason why you dont work with the master branch?

[songheony]

Thank you for your reply.
Because of some reason, I couldn't use PyTorch 1.0 but 0.4.1. But in your master branch, the code requires torchvision >= 0.3.0 and it is not compatible with PyTorch 0.4.1. I think there are some people who are in same situation as me. Moreover, to reproduce your paper, I think we need to use this branch.