[`Blip`] Fix slow tests and doctests with correct values
# What does this PR do?

Fixes https://github.com/huggingface/transformers/issues/22625

In fact, the `num_attention_heads` of `BlipTextModel` should be 12 and not 8. Hence the models that are on the Hub were producing different logits / generations that the original implementation in some cases.

I made PRs on the Hub:

- https://huggingface.co/Salesforce/blip-itm-large-flickr/discussions/1
- https://huggingface.co/Salesforce/blip-itm-base-coco/discussions/3
- https://huggingface.co/Salesforce/blip-image-captioning-base/discussions/13
- https://huggingface.co/Salesforce/blip-image-captioning-large/discussions/8#642eed4cce2efe48a1aa1497
- https://huggingface.co/Salesforce/blip-vqa-base/discussions/3#642eed68ae8ae35b7a9bbb7f
- https://huggingface.co/Salesforce/blip-vqa-capfilt-large/discussions/5
- https://huggingface.co/Salesforce/blip-itm-large-flickr/discussions/3
- https://huggingface.co/Salesforce/blip-itm-large-coco/discussions/3

And tested them on the slow tests and doctests with `.from_pretrained(xxx, revision="refs/pr/xx")`, this PR fixes the slow tests and doctests with the correct values. Let's merge this PR and I'll merge the PRs that are on the Hub

cc @sgugger 

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