fix: (issue #32689) AttributeError raised when using Trainer with eval_on_start=True in Jupyter Notebook.

[fshp971]

What does this PR do?

Fixes #32689 (issue)

The cause of issue #32689 is that in Jupyter Notebook, for a transformers.Trainer, when its argument eval_on_start is set to True, running Trainer.train() will lead to the following function execution order:

1. NotebookProgressCallback.on_train_begin()
2. NotebookProgressCallback.on_prediction_step()
3. NotebookProgressCallback.on_train_end()

Running NotebookProgressCallback.on_prediction_step() directly after NotebookProgressCallback.on_train_begin() will result in two attributes of NotebookProgressBar, i.e., value and label, being used before assigned (see methods update(), update_bar() and display() of NotebookProgressBar in transformers/utils/notebook.py for details).

A very simple solution is to initialize both attributes value and label to None during the creation of NotebookProgressBar.

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@amyeroberts @ArthurZucker

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[muellerzr]

Nice and simple solution!

[HuggingFaceDocBuilderDev]

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[ArthurZucker]

🤗