Fix undefined default_config in configuration_utils.py
Author: mgoin
Pull request: yes

# What does this PR do?

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Fixes https://github.com/huggingface/transformers/issues/33927


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Comments:
cc @gante 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_33934). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.