Fix key mapping for VLMs
Author: bvantuan
Pull request: yes
Labels: name: for patch

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Fixes #39004
Fixes key mapping for VLMs when they are extended by a custom subclass.


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Comments:
Hi @zucchini-nlp ! Thank you for reviewing my PR. Yes, I’ve also updated the corresponding line in `save_pretrained` 👍.

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_39029). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

Insane PR, thanks a lot @bvantuan, wasn't aware of mro[:-1] when I proposed the key mapping patch !