GEN refactoring

[mariadalfonso]

Refactoring started here, need to be tested extremely well

• [ ] master https://github.com/cms-sw/cmssw/pull/32167

[vlimant]

we will keep track of things for V11 in the googledoc : https://docs.google.com/document/d/1UaAEmLBbA8upMzIZ4Rnv5__ychOsEVBDecca0zHAQ4I/edit?usp=sharing