Make CLIP model could use new added tokens with meaningful pooling
# What does this PR do?

Fix #24650

This is to address the feature request #24650.

Although we have default values of bos/eos been corrected in #24773, the existing config on the Hub still have the incorrect value `1` and `2`, which prevents CLIP model to use new added tokens when a user add them.

Although we can open mass PRs on the Hub, I want to decouple (slightly) this with the ability to support such feature.
With this PR, if a user want to use new added tokens, they has to specify/update the `eos_token_id`.

**We don't need to wait all Hub repo. to be updated to merge this PR.**


I will fix the CI by using `fix-copies` later<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>I will merge once the branch is cut tonight.<|||||>Sorry for the spam: @sgugger said the branch cut would be on next Monday. I think it's safer to wait until then.