[Switch Transformers] Fix failing slow test
# What does this PR do?

This PR should fix the currently failing slow test for `SwitchTransformers`.
Putting the model on GPU and running the inference on GPU should fix the test. I have tested the test on PyTorch 1.12 (same as the daily CI runner), but I need to test it on the daily CI runner first before merging

cc @ydshieh @ArthurZucker @sgugger 

EDIT: it passes on the CI daily runner, marking it as ready for review
_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks very much for the explanation! This should have been fixed now 💪 