sudo rosdep init
ERROR: cannot download default sources list from:
https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/sources.list.d/20-default.list
Website may be down.

reference:
https://zhuanlan.zhihu.com/p/77483614
https://blog.csdn.net/gddxz_zhouhao/article/details/72861266