Fix type issue in using bucketing with Trainer
Author: seopbo
Pull request: yes

# What does this PR do?

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- Fix type issues in `LengthGrouperSampler`, `DistributedLengthGroupedSampler`

Fixes #18003


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@sgugger 

Comments:
_The documentation is not available anymore as the PR was closed or merged._

> Oh and sorry I didn't catch it at the first review, but we can group the else if in an elif.

I totally agree this. @sgugger 

Thanks a lot!