[`Blip2`] Add int8 support for `blip2-flan-t5-xxl` 
# What does this PR do?

Similarly as https://github.com/huggingface/transformers/pull/20683 - currently the largest blip2 + flan-t5-xxl fails to run in int8 since we forgot to add the patch presented in the aforementioned PR

This PR adds the `_keep_in_fp32_modules` support for `Blip2` so that users can run this model in fp16 and int8 with no performance degradation as the original model trained in bf16

cc @sgugger @NielsRogge 

_The documentation is not available anymore as the PR was closed or merged._