[PT2][Quant][BE] Test refactor to be organize them better (#102704)
Collected most of the test modules under TestHelperModules. This allows reuse
of modules when possible. Probably we can refactor a bit more but left some qat
related helper modules in their respective tests
Differential Revision: [D46267687](https://our.internmc.facebook.com/intern/diff/D46267687/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/102704
Approved by: https://github.com/andrewor14
