fix backend check (#2670)
* fix backend check
* reformat backend check
* Update src/accelerate/state.py
Co-authored-by: Zach Mueller <muellerzr@gmail.com>
* Update src/accelerate/state.py
Co-authored-by: Zach Mueller <muellerzr@gmail.com>
* raise value error if backend mismatch
* Update src/accelerate/state.py
Co-authored-by: Zach Mueller <muellerzr@gmail.com>
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Co-authored-by: Zach Mueller <muellerzr@gmail.com>
