Use trl's guarded is_liger_kernel_available in DPOTrainer

[qgallouedec]

Part of the effort to align DPOTrainer and KTOTrainer (experimental). #4786

DPOTrainer imported is_liger_kernel_available from transformers.utils, which only checks that liger is installed. Switched it to trl.import_utils.is_liger_kernel_available, which additionally enforces the minimum supported version (liger ≥ 0.8.0, LIGER_KERNEL_MIN_VERSION).

This matches KTOTrainer and GRPOTrainer, which already use the guarded version.

No behavior change beyond rejecting liger installs older than the supported minimum.

---

Note

Low Risk
Import-only alignment with existing TRL trainers; behavior changes only for liger versions below 0.8.0, which were previously allowed but may be unsupported.

Overview
DPOTrainer now imports is_liger_kernel_available from trl.import_utils instead of transformers.utils, matching KTOTrainer and GRPOTrainer.

TRL’s helper still requires liger_kernel to be installed, and additionally treats versions below 0.8.0 (LIGER_KERNEL_MIN_VERSION) as unavailable. That affects the conditional import of LigerFusedLinearDPOLoss and the use_liger_kernel=True guard in __init__: environments with an older liger install will no longer enable the fused Liger DPO path.

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