Remove timm backbone weight loading to prevent meta-tensor warnings in `from_pretrained`
Author: HichTala
Pull request: yes

# What does this PR do?

This pull request makes a targeted change to the model loading logic in `modeling_utils.py` to improve compatibility with timm backbones. Specifically, it ensures that when a model is configured to use a timm backbone with pretrained weights (`use_pretrained_backbone=True`), the backbone weights are not loaded while the model is still in `meta` device during `from_pretrained()`.

**Model initialization and configuration:**

* In `from_pretrained`, before model instantiation, if the config specifies both `use_timm_backbone` and `use_pretrained_backbone`, the code sets `config.use_pretrained_backbone` to `False` to defer loading pretrained timm backbone weights until after initialization.



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Fixes # (issue)
- #37615 

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## Who can review?

@Rocketknight1 @qubvel @CyrilVallez @NielsRogge @xenova 

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Comments:
Hey @HichTala! Are you sure you are still facing the original issue in latest `main`? Loading has changed, and we do not use `model.load_state_dict` anymore!

Hi @Cyrilvallez! I just tested this again with the latest `main`, and the original issue is still present.

The problem is not related to `load_state_dict` itself. It comes from the fact that during `from_pretrained`, when a model uses a `timm` backbone with `use_pretrained_backbone=True`, the model is still instantiated on meta devices, but the `timm` pretrained backbone weights are already being loaded at that stage. This causes all the warnings from `timm`, and then, once `from_pretrained` continues, those backbone weights are overwritten anyway by the Hugging Face checkpoint loading logic.

This PR simply removes loading the `timm` backbone weights until after the model is fully materialized in `from_pretrained` method.

I see... Then this config switch should be hardcoded in all model config when this can happen, not in the general loading code! Do you want to make the change?

Yep totally agree with that, I'm making the change!

**[For maintainers]** Suggested jobs to run (before merge)

run-slow: conditional_detr, dab_detr, deformable_detr, detr, table_transformer

Hi, sorry for the delay, I completely forgot about this PR. Here are the updates

Hey! I believe this PR is superseded by https://github.com/huggingface/transformers/pull/43323

Ok cool I'm closing this then

superseded by https://github.com/huggingface/transformers/pull/43323