ModuleNotFoundError: No module named 'mmcv._ext'

ModuleNotFoundError: No module named 'mmcv._ext'

If you are getting this error when trying to run the latest version of mmdet with the latest version of mmcv (installed via pip), this solves the problem 
pip uninstall mmcv mmcv-full
git clone https://github.com/open-mmlab/mmcv.git
cd mmcv
MMCV_WITH_OPS=1 pip install -e .