Move _get_kl_completion_ids into _get_kl_dataset

[qgallouedec]

_get_kl_completion_ids was a module-level helper used in exactly one place. Move it into its only caller, KTOTrainer._get_kl_dataset, as a nested function, alongside the existing rename_kl_fn.

The direct unit test that imported the helper is removed; its behavior stays covered through the trainer's KL-dataset preparation.

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Note

Low Risk
Pure structural refactor with identical KL dataset mapping logic; only test surface area shrinks for the former public helper.

Overview
Refactors where KL mismatching is defined: the module-level _get_kl_completion_ids helper is removed and the same batched completion cycling logic lives as a nested get_kl_completion_ids inside KTOTrainer._get_kl_dataset, next to rename_kl_fn. The vision collator comment now points at that nested name.

Tests drop the standalone unit test that imported _get_kl_completion_ids and asserted rotation on a synthetic Dataset; imports are trimmed accordingly. KL pairing behavior is unchanged and remains covered via trainer/collator paths (e.g. test_kl_cycling, end-to-end KTO training).

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