Add support for DetectoRS. (#3064)

* Add support for DetectoRS.

* Fix Travis CI build failure and remove loading rfp_pretrained for test

* Revise the code according to the PR reviews:

* remove unnecessary `,`

* add docstring to ASPP and ConvAWS2d

* make args of ASPP configurable

* simplify the code of ConvAWS2d

* import nn and functional for SAConv2d

* rewrite rfp_conv with build_conv_layer and constant_init

* rename rfp_inp with rfp_inplanes

* reformat code to pass Travis CI build

* Revise codes according to the PR reviews:

* Create new backbone DetectoRS_ResNe(X)t to incorporate RFP and SAC.

* Initialize modules with mmcv.cnn.

* Revise the codes of RFP.

* Revise codes according to the PR comments:

* Add docstrings to Bottleneck in mmdet/models/backbones/detectors_resnet.py.

* Add docstrings to _load_from_state_dict in mmdet/ops/conv_ws.py.

* Initialize the switch of SAC with mmcv.

* minior refactor and docstring

* update doc

* update README

* move init into init_weights()

* update model zoo

* add gfl

Co-authored-by: Jiarui XU <xvjiarui0826@gmail.com>