Generate: Enable easier TextStreamer customization

[vblagoje]

What does this PR do?

Minimally adapts recently integrated (#22449) by adding a more obvious API hook for streaming tokens yet retains all the current semantics.

I am excited to use TextStreamer but I found the hook-in API not as intuitive as it could be if one needs to customize token printing. For example, I need to use specific colouring to print arriving tokens, yet achieving this without a complete TextStreamer rewrite is not as easy. We can easily create an obvious hook-in method:

def on_new_token(self, token: str, stream_end: bool = False):

This method is called by TextStreamer yet its subclasses can easily customize printing. The default implementation of on_new_token method simply prints tokens to stdout as it currently does.

I don't foresee any major documentation updates as a consequence of this PR.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@gante @sgugger

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[gante]

I like the structure, but I dislike the name on_new_token. It's actually a callback on "new print-ready text". Perhaps on_finalized_text? WDYT?

[vblagoje]

I like the structure, but I dislike the name on_new_token. It's actually a callback on "new print-ready text". Perhaps on_finalized_text? WDYT?

@gante I am completely indifferent regarding the name. Please adjust the name as you like!
On a second look, I'm also not 100% sure whether to put this method into TextStreamer only. Your call.

[gante]

Let's keep it in TextStreamer for now. It's still early to tell how people will want to use it :)

[gante]

LGTM

(@sgugger with this new fn, the newer iterator streamer can inherit from the stdout streamer, sharing the text preparation logic)

[sgugger]

LGTM, thanks for your contribution!