Move common properties to BackboneMixin
# What does this PR do?

First of a series of PRs to enable loading timm checkpoints using the `AutoBackbone` API. This PR moves common logic e.g. `channels` property to the `BackboneMixin` class. This is for two main reasons: 
* Reduce duplicated code
* Enable using similar logic across the transformer and timm backbones


## Series of PRs
- [x] Moving common logic into the `BackboneMixin` class (this PR)
- [ ] Add `out_indices` to backbones  - [PR](https://github.com/amyeroberts/transformers/pull/109/files)
Note: This is an optional design choice and not necessary for loading the timm backbones
- [ ] Add tests for backbone models - [PR](https://github.com/amyeroberts/transformers/pull/110/files) 
- [ ] Add `TimmBackbone` model that can be loaded through `AutoBackbone`  - [PR](https://github.com/amyeroberts/transformers/pull/111/files)
This is where all the important stuff happens 🪄 

## 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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- [ ] Did you write any new necessary tests?
_The documentation is not available anymore as the PR was closed or merged._