Resolve todo and fix docstrings in fastaiv2 example

[nzw0301]

Motivation

Resolve the left TODO task of the fastaiv2 example.

Description of the changes

• Remove lines by following instructions in the comment
• Fix docstring by following coding style of Optuna

[toshihikoyanase]

@gen740 Could you review this PR? Please feel free to remove the assignment if you are busy. CC @HideakiImamura

[github-actions[bot]]

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