[bnb] Let's warn users when saving 8-bit models (#20282)
* add warning on 8-bit models
- added tests
- added wrapper
* move to a private attribute
- remove wrapper
- changed `save_pretrained` method
* Apply suggestions from code review
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
* fix suggestions
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
