[Trainer] add error when passing `8bit`models
# What does this PR do?

Before this PR, any user could load an 8bit model and pass it to a Trainer, which is wrong. In fact, it is not possible to train an 8bit model (yet). Therefore we should raise an error until this will be supported in the future

Related: https://github.com/huggingface/transformers/issues/20348#issuecomment-1335106257

cc @ydshieh @sgugger 


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