Description
TLDR: run git clean -xi and remove all files that you didn't create yourself, then re-install, or use snippet below.
If you're contributing a PR and you recently merged upstream/master, you should see a lot untracked files when running git status.
This is normal: these files were previously automatically generated and added to the .gitignore. They have now been taken out of the gitignore (#17132) and can be safely removed.
To remove them, you can use git clean -xi. This will also remove installation files like .so, dll or .c files, so you'll finally need to re-install scikit-learn by running e.g. pip install -e . or make in.