Fixes a useless warning in `generate`.
Author: Narsil
Pull request: yes

# What does this PR do?

We removed `max_length` as a necessary parameter for `greedy`/`sample`/`beam_search`/`group_beam_search`.

Unfortunately a extra warnings made its way in the PR which is unwarranted for.

Fixes #11371
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Fixes # (issue)


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## Who can review?

@stas00 @LysandreJik


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Comments:
Sorry about that ! 

I'll wait for @patrickvonplaten before merging.