Instance generators difficulty fixes

[dchetelat]

Pull request checklist

• [x] I have opened an issue to discuss the proposed changes. (Fixes #273.)
• [ ] I have modified/added tests to cover the new changes/features.
• [ ] I have modified/added the documentation to cover the new changes/features.
• [x] I have ran the tests, checks, and code formatters.

Proposed implementation

[AntoinePrv]

@dchetelat can you confirm the rebase did not mess up anything