[`InstructBlip`] Add instruct blip int8 test
# What does this PR do?

Addresses: https://github.com/huggingface/transformers/pull/24490#discussion_r1242098216

Also fixes an inconsistency with Blip / Blip2 processors in order for users to be able to call `.to()` with both device and dtype argument to cast the input in half precision if necessary. Happy to move that in another PR if needed

cc @sgugger @ydshieh 

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