If you mean with signing out signing out of the SSO Provider, than a redirect to yourappurl/SSO/logout can do the trick.
https://forum.mendix.com/link/space/connectors/questions/88476
Otherwise what is the usecase of signing out of the app, but not from the SSO provider….Only as a developer I miss that sometimes, but then I navigtae to myapp/login.html and login with a non sso user...