Avoid zero or too small value for beta in dynamic rcnn (#4303)
Avoid zero or too small value for beta in dynamic rcnn (#4303)

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

* undo

* fix #3950, avoid zero or too small value for new_beta

* make eps as global var
open-mmlab/mmdetection,open-mmlab/mmdetection