fix: prevent model access error during Optuna hyperparameter tuning

[emapco]

What does this PR do?

The transformers.integrations.integration_utils.run_hp_search_optuna function releases model memory and sets trainer.model to None after each trial. This causes an AttributeError when subsequent Trainer.train calls attempt to access the model before reinitialization. This is only an issue when fp16_full_eval or bf16_full_eval flags are enabled.

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Who can review?

@SunMarc

[emapco]

@SunMarc My bad for closing the previous issue. I included a test that targets the specific conditions that trigger the issue.

[SunMarc]

Thanks for the test ! Just a nit

[HuggingFaceDocBuilderDev]

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