Remove unused `cur_len` in generation_utils.py
Author: ekagra-ranjan
Pull request: yes

# What does this PR do?

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Removes unused `cur_len` in `generation_utils.py`


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## Who can review?

@patrickvonplaten @gante 

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

Hi @patrickvonplaten - please review this one when you get some time.

Nice!