Add warning in `generate` & `device_map=auto` & half precision models (#19468)
* fix device mismatch
* make fixup
* added slow tests
- added slow tests on `bnb` models to make sure generate works correctly
* replace with `self.device`
* revert force device assign
* Update src/transformers/generation_utils.py
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
* set the warning in `generate` instead of `sample`
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>