[OPT] Run test in lower precision on GPU
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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Let's run the test only in half precision. There might have been a bug because fp16 weights are loaded into fp32. Let's make sure everything stays in fp16.

Also if it fails again, we'll see a better error message this time.

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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Models:

- albert, bert, xlm: @LysandreJik
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