Optuna: Show title in figure for contour plot.

[himkt]

I noticed optuna.visualization.plot_contour show a plot without its title.

I think it's better to show a title to be consistent with other plot features such as optuna.visualization.plot_optimization_history and optuna.visualization.plot_slice.

We can reproduce this behavior in the official quickstart

Expected behavior

Show title of a figure in the contour plot.

Environment

• Optuna version: 1.3.0
• Python version: 3.6.9
• OS: Ubuntu 18.04.3 LTS

Steps to reproduce

Run notebook on Google Colab.

[harupy]

Can I work on this?

[himkt]

@harupy Sure, thank you for your interest!