Encapsulate DCN into a ConvModule & Conv_layers (#1894)

* update code

* reformat

* fix load pretrain bug

* reformat

* fix key warnings

* use build_conv_layer & ConvModule

* rm unrelated file

* fix resnext group bug

* change cfg to pass test

* use build_conv_layer to claim dcn in resnets

* change _version = 2

* MDCN to DCNv2

* change dependency of mmcv from 0.2.15 to 0.2.16

* resolve comments

* rm comments

* get_root_logger()