Align KTO with DPO: Align Liger loss naming

[albertvillanova]

Align KTO with DPO: Align Liger loss naming.

Follow-up to:

• Align Liger loss naming across core trainers #6243

Part of:

• KTO refactoring to be promoted to stable #4786

This PR refactors the naming of the Liger loss function attribute in the KTOTrainer class to improve code clarity and consistency. The main change is renaming the attribute from liger_loss_fn to liger_loss and updating its usage accordingly.

Changes

Refactoring for clarity:

• Renamed the Liger loss function attribute from liger_loss_fn to liger_loss in the KTOTrainer class constructor (__init__) for clearer and more consistent naming.
• Updated all internal references to use the new attribute name liger_loss instead of liger_loss_fn in the _compute_loss_liger method.

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Note

Low Risk
Identifier rename only; no logic, API, or training behavior changes.

Overview
Renames the Liger fused KTO loss attribute on KTOTrainer from liger_loss_fn to liger_loss, and updates the call site in _compute_loss_liger to match. This is a naming-only change so KTO follows the same convention as DPO after #6243; behavior is unchanged.

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