Summary:
Based on orionr's work

 - [x] Solve [the problem about GenerateProposals](https://github.com/facebookresearch/Detectron/pull/334#issuecomment-378764468)
 - [x] Use the existing [ResizeNearest](https://github.com/caffe2/caffe2/blob/master/caffe2/operators/resize_op.cc#L57) layer instead of UpsampleNearest. ResizeNearest has cpu implementation and neon optimization
 - [x] Make it work (with https://github.com/pytorch/pytorch/pull/7091)

With this PR, FPN is supported in cooperation with https://github.com/pytorch/pytorch/pull/7091. I have verified that it works on `e2e_faster_rcnn_R-50-FPN_1x.yaml`
Pull Request resolved: https://github.com/facebookresearch/Detectron/pull/372

Reviewed By: newstzpz

Differential Revision: D9213242

Pulled By: rbgirshick

fbshipit-source-id: 8fc7b77e6cbf08adaafd760505dd760df59bfd79