Fix GradScaler deprecation warning

[a-r-r-o-w]

What does this PR do?

Currently, using GradScaler raises the following warning on the latest torch nightly:

# /raid/aryan/nightly-venv/lib/python3.10/site-packages/accelerate/accelerator.py:494: FutureWarning: `torch.cuda.amp.GradScaler(args...)` is deprecated. Please use `torch.amp.GradScaler('cuda', args...)` instead.

This is due to it being deprecated in pytorch/pytorch#126527. Just a small fix with the new intended usage.

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@muellerzr @SunMarc

[muellerzr]

Thanks for catching this!

[SunMarc]

Nice !