Fix Blip-2 CI
# What does this PR do?

Avoid GPU OOM by using FP16.
_The documentation is not available anymore as the PR was closed or merged._<|||||>since we have put `_keep_in_fp32_modules` [in a previous PR](https://github.com/huggingface/transformers/pull/21574), I think it should work!<|||||>@sgugger I never merge my PR without running on a CI runner-like machines: ran it 3 times and all pass.<|||||>Yes I think we can be confident to say that @sgugger 