Fix validation when accelerator is a string

[awaelchli]

What does this PR do?

Fixes #12698

Choosing accelerator="cocofruit" would silently convert it to "cpu".
Now, we will raise an error. See the added test.

Does your PR introduce any breaking changes? If yes, please list them.

No.

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cc @Borda @justusschock @kaushikb11 @awaelchli @ninginthecloud @akihironitta @rohitgr7

[tchaton]

Nice catch !