Fix: NotebookProgressCallback crash when evaluating with the Trainer

[Charly21r]

What does this PR do?

Fixes #44936

This PR fixes an issue with NotebookProgressCallback in the Trainer where calling evaluate() before or after training would crash due to the training tracker being None. The callback now properly handles evaluation even if training has not yet started or if it has already finished, ensuring metrics can be computed and displayed.

Previously, the on_evaluate method assumed that self.training_tracker was always initialized, but:

• Before training: self.training_tracker has not being initialised by on_train_begin yet.
• After training: on_train_end sets self.training_tracker to None, so calling on_evaluate afterwards would fail.

Fix: on_evaluate now checks whether self.training_tracker exists before using it, and safely handles cases where it is None. This prevents crashes and ensures evaluation can run regardless of training state.

Additionally, new unit tests were added to ensure that evaluation works in this scenario, and existing notebook callback tests were updated to cover this case. This improves robustness of notebook-based workflows, especially in Jupyter or Colab environments.

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Who can review?

@SunMarc

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[SunMarc]

does this fix your issue @HenrikEilers ?