[Doctest] Add `configuration_encoder_decoder.py`
Author: SrijanSahaySrivastava
Pull request: yes

# What does this PR do?

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Fixes #19487 


## 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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Models:

- text models: @ArthurZucker and @younesbelkada
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Library:

- flax: @sanchit-gandhi
- generate: @gante
- pipelines: @Narsil
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- tokenizers: @ArthurZucker
- trainer: @muellerzr and @pacman100

Integrations:

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Comments:
@ydshieh please look into this. I'm to this.

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_26519). All of your documentation changes will be reflected on that endpoint.