Update docstring for CLIPConfig
Author: yingzha
Pull request: yes

# What does this PR do?

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Fixes missing imports in CLIPConfig docstring. 


## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


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Models:

- text models: @ArthurZucker and @younesbelkada
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Comments:
_The documentation is not available anymore as the PR was closed or merged._

Yes, you will need to refresh your circleCI permissions and push an empty commit so we can check the tests are passing.

@sgugger Done.