问题：
在安装 mmcv 的时候使用 pip install mmcv 报错如下：
WARNING: Retrying (Retry(total=4, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError(': Failed to establish a new connection: [Errno -2] Name or service not known',)': /simple/mmcv/
解决方案：
输入如下代码：
sudo echo "nameserver 8.8.8.8" | tee /etc/resolv.conf > /dev/null
Collecting mmcv
Downloading mmcv-1.6.1.tar.gz (563 kB)
|████████████████████████████████| 563 kB 986 kB/s
Collecting addict
Downloading addict-2.4.0-py3-none-any.whl (3.8 kB)
Requirement already satisfied: numpy in /usr/local/lib/python3.8/dist-packages (from mmcv) (1.19.5)
Requirement already satisfied: packaging in /usr/local/lib/python3.8/dist-packages (from mmcv) (20.9)
Requirement already satisfied: Pillow in /usr/local/lib/python3.8/dist-packages (from mmcv) (8.1.2)
Requirement already satisfied: pyyaml in /usr/local/lib/python3.8/dist-packages (from mmcv) (5.4.1)
Collecting yapf
Downloading yapf-0.32.0-py2.py3-none-any.whl (190 kB)
|████████████████████████████████| 190 kB 31.8 MB/s
Requirement already satisfied: pyparsing>=2.0.2 in /usr/local/lib/python3.8/dist-packages (from packaging->mmcv) (2.4.7)
Building wheels for collected packages: mmcv
Building wheel for mmcv (setup.py) ... done
Created wheel for mmcv: filename=mmcv-1.6.1-py2.py3-none-any.whl size=860277 sha256=4f25550e01acc52694ddc0d12451bd57a9b2dd7f78f3cc679e54687323517eeb
Stored in directory: /root/.cache/pip/wheels/e3/ac/32/75552e05d1964b2123f074fe3e87c890e9ac83bc2d2c5b59d6
Successfully built mmcv
Installing collected packages: yapf, addict, mmcv
Successfully installed addict-2.4.0 mmcv-1.6.1 yapf-0.32.0
