Standardize TrainerCallback import to the public top-level path

[qgallouedec]

Standardize every TrainerCallback import across the repo to the public API form from transformers import TrainerCallback.

No behavior change.

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Note

Low Risk
Import path only; no runtime or training logic changes.

Overview
TrainerCallback is now imported from the public transformers package (from transformers import …, TrainerCallback) instead of the internal transformers.trainer_callback path in tpo_trainer.py, dpo_trainer.py, and sft_trainer.py.

This is an import-only cleanup; trainer behavior and callback typing are unchanged.

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[bot-ci-comment]

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