Reimplement "FCOS: Fully Convolutional One-Stage Object Detection" (#586)

* add fcos

* use P5 instead of C5

* add relu before extra convs in FPN

* add singleclass_nms, use caffe2 lr

* fix log interval

* use caffe2init and relu in extra layers

* fix scale layer, use p5 instead of c5

* fix focs target

* refactor code

* delete useless file

* clean

* refactor code

* change num_classes to cls_out_channels

* fix bug of in get_bboxes

* fix bug in test

* add r101 2x cfg

* ms use value mode, add x101-64x4d cfg

* add more comment and rename some variable

* rename centers to points, modify doc string of distance2bbox

* add fcos detector, replace frozen with requires_grad

* add README.md

* add r101-1x performance, rename cfg, add detector FCOS

* update fcos r50 2x performance, remove fpn caffe2 initialize

* fix flake8 error

* rename cfg

* fix grammar error of some comments

* minor fix comment

* change work_dir to be consistent with config name

* add FCOS support in README