Remove redundant backend checks in training_args.py
Author: kevint324
Pull request: yes


# What does this PR do?

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Fixes  #28109



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Models:

- text models: @ArthurZucker and @younesbelkada
- vision models: @amyeroberts
- speech models: @sanchit-gandhi
- graph models: @clefourrier

Library:

- flax: @sanchit-gandhi
- generate: @gante
- pipelines: @Narsil
- tensorflow: @gante and @Rocketknight1
- tokenizers: @ArthurZucker
- trainer: @muellerzr and @pacman100

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- deepspeed: HF Trainer/Accelerate: @pacman100
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- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

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@ArthurZucker @muellerzr 

Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_30999). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

Thanks @kevint324! Overall this looks fine but let's fix the failing quality checks!

```bash
pip install -e .[quality]; make fixup;
```

@muellerzr 
Done. After `make fixup;`  a extra import is removed.

Thanks

I looked into the failed case and found the casue.

In my PR I removed the calls to self.device within the `if` statements which calls `_setup_devices` to do some initialization.



I've no idea why [test_offline.py : test_offline_mode](https://app.circleci.com/pipelines/github/huggingface/transformers/94104/workflows/61f0aecd-d79b-4775-8030-4a8d4a7bf9e9/jobs/1235408) still fails. 

I tried it on main and it still failed. @ArthurZucker 


Failing tests are independent of the changes, merging