Add TimmBackbone model
# What does this PR do?

Adds a new model TimmBackbone to use for loading timm weights for use in the AutoBackbone API.

Example usage: 
```
from transformers import AutoBackbone

# Loads a transformers model 
backbone = AutoBackbone.from_pretrained("microsoft/resnet-18")

# Loads a timm checkpoint
backbone = AutoBackbone.from_pretrained("resnet18", use_timm_backbone=True)
```

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
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      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?
_The documentation is not available anymore as the PR was closed or merged._<|||||>@ydshieh Merging now. I'll address any comments or requests for changes in a follow up PR.