[`DETR`] Remove timm hardcoded logic in modeling files
Author: amyeroberts
Pull request: yes

# What does this PR do?

Certain models use timm's `create_model` to load their backbone.

In future, all models should use `load_backbone` to create the backbone, removing the need for the conditional timm logic. Removing this from existing models isn't possible, because it changes the weight names for the backbone as the backbone is now loaded as a TimmBackbone class i.e. existing checkpoints wouldn't compatible. 

This PR makes it possible to configure the timm backbone loaded completely through the model config, removing the hard-coded values in the modeling files. So, for users, it's the same as-if `load_backbone` was being used. 


## 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.
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- [ ] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

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Comments:
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