[`from_pretrained`] Simpler code for peft (#25726)
* refactor complicated from pretrained for peft
* nits
* more nits
* Update src/transformers/modeling_utils.py
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
* make tests happy
* fixup after merge
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Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>