Fix trainer test wrt DeepSpeed + auto_find_bs

[muellerzr]

What does this PR do?

Follow-up to #29057, changes the test to ensure it raises a not-implemented-error

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Will verify on our CI before merging (as local deepspeed won't install for me).

Can confirm passes: https://github.com/huggingface/accelerate/actions/runs/7932048373

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[muellerzr]

cc @amyeroberts good for review now that I verified tests pass :)

[amyeroberts]

Thanks for fixing!