Goodbye to Blip-2 doctests
# What does this PR do?

Blip-2 causes GPU OOM on doctest, and potentially causes some subsequential tests failing (as the GPU is not in good state, but I didn't verify thoroughly).

This PR tries to avoid this situation, despite the added lines is not really meant to be in a doc for users.
> I don't think the doctests should run on a 2.7b-parameter model, that's not a good use of our resources. So the doctest should be skipped if there is not a smaller checkpoint available.

It could be run with FP16 + the changes in this PR. In terms of resources, the machine is always on, so I don't see the waste. Could you elaborate a bit more regarding your thought on the resources (while we can make it fit into the T4 GPUs eventaully)<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>OK, I agree! Will skip<|||||>cc @NielsRogge for information<|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_21566). All of your documentation changes will be reflected on that endpoint.