Support R50 and R101 pretrained PAA detectors for multi-scale 3x training schedules (#4495)

* fix #4181, invalid link in readme for maskrcnn cityscapes

* undo

* add config for paa_r50_fpn_mstrain_3x_coco.py

* reformat for CI building

* add config paa_r101_fpn_mstrain_3x_coco.py

* add r50 and r101 multiscale training 3x performance

* add pretrained model link