[ao] making _is_activation_post_process private with BC (#90554)
same function in observer and quantize, consolidated to a
single function
note: this is a recreation of D40709276 which caused severa breakages due to not maintaining BC for models with cached code with calls to the old function name
Differential Revision: [D41793604](https://our.internmc.facebook.com/intern/diff/D41793604/)
**NOTE FOR REVIEWERS**: This PR has internal Meta-specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D41793604/)!
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90554
Approved by: https://github.com/jcaip
