[bnb] Let's warn users when saving 8-bit models
# What does this PR do?

This PR warns users that try to save 8-bit loaded models

closes #20247
closes #19480

cc @sgugger 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20282). All of your documentation changes will be reflected on that endpoint.<|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20282). All of your documentation changes will be reflected on that endpoint.<|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20282). All of your documentation changes will be reflected on that endpoint.<|||||>Ah ah made a typo in one of my suggestions, sorry. `getattr` has less `t`s ;-)<|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20282). All of your documentation changes will be reflected on that endpoint.<|||||>Thanks for implementing this. This fix would absolutely have prevented the confusion explained in: [#20247](https://github.com/huggingface/transformers/issues/20247).<|||||>Thank you very much @peregilk ! 🙏 