Add bitsandbytes support for gpt2 models (#24504)
* Add bitsandbytes support for gpt2 models
* Guard Conv1D import to pass tensorflow test
* Appease ruff linter
* Fix 4bit test and remove int8 test boilerplate
* Update tests/bnb/test_mixed_int8.py
Co-authored-by: Younes Belkada <49240599+younesbelkada@users.noreply.github.com>
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Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Younes Belkada <49240599+younesbelkada@users.noreply.github.com>
