Convert `torch_dtype` as `str` to actual torch data type (i.e. "float16" …to `torch.float16`)
Author: KossaiSbai
Pull request: yes

# What does this PR do?

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Fixes [#27087](https://github.com/huggingface/transformers/issues/27087)


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Comments:
@amyeroberts committed your suggestion, thanks for making me understand the difference between `is` and `isinstance`

@KossaiSbai There's still some failing tests on the CI run. Rebasing on the most recent version of main should resolve this

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_28208). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

@amyeroberts I have just merged my branch into master, let's see if the CI tests now pass