Fix/best model checkpoint fix (#35885)
* Set best_model_checkpoint only when ckpt exists.
Rather than set it explicitly without checking if the checkpoint directory even exists as before, now we moved the setting logic inside of _save_checkpoint and are only setting it if it exists.
* Added best_global_step to TrainerState.
* Added tests for best_model_checkpoint.
* Fixed hard-coded values in test to prevent fail.
* Added helper func and removed hard-coded best_step.
* Added side effect patch generator for _eval.
* Added evaluate side effect func.
* Removed erroneous patching.
* Fixed minor bug.
* Applied Ruff.
* Fixed Ruff problem in make style.
* Used Trainer.set_initial_training_values.
