Remove unused flush_right

[albertvillanova]

Remove unused flush_right.

Follow-up to:

• Remove post-collation truncation from DPO #5350

This PR removes the flush_right utility function and its associated tests from the codebase. This simplifies the utilities and test files by eliminating unused or unnecessary code.

Changes

Removals related to flush_right:

• Removed the flush_right function from trl/trainer/utils.py.
• Deleted the TestFlushRight test class and all related test cases from tests/test_utils.py.
• Removed the import of flush_right from tests/test_utils.py.

---

Note

Low Risk
Low risk cleanup that removes an unused utility and its unit tests; behavior is unchanged unless downstream code relied on the previously exported symbol.

Overview
Removes the unused flush_right helper from trl/trainer/utils.py and drops its import and the entire TestFlushRight test suite from tests/test_utils.py.

This PR is purely a cleanup to reduce dead utility/test code and simplify the utils surface area.

^{Written by Cursor Bugbot for commit 7ecc924. This will update automatically on new commits. Configure here.}

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.