Export base streamer.
Author: AndreasAbdi
Pull request: yes

Previously, the base streamer class was not exported so the set of available streamers was fixed to 3 streamer classes. 

This change makes it so that customers may extend the default base streamer class.

# What does this PR do?
Previously, the base streamer class was not exported so the set of available streamers was fixed to 3 streamer classes. 

This change makes it so that customers may extend the default base streamer class.


Fixes # (issue)

None
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      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [X ] Did you write any new necessary tests?


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Comments:
Hi 👋, thank you for opening this pull request! The pull request is converted to draft by default. When it is ready for review, please click the `Ready for review` button (at the bottom of the PR page).

cc @gante

great thanks!