Plot contour graph in backend

[knshnb]

Contributor License Agreement

This repository (optuna-dashboard) and Goptuna share common code.
This pull request may therefore be ported to Goptuna.
Make sure that you understand the consequences concerning licenses and check the box below if you accept the term before creating this pull request.

• I agree this patch may be ported to Goptuna by other Goptuna contributors.

Reference Issues/PRs

We experimentally introduce a backend rendering of graphs by using optuna.visualization as a possible substitution for frontend rendering, which is becoming a large codebase and is getting hard to maintain.
Ref: #156

What does this implement/fix? Explain your changes.

Use optuna.visualization.plot_contour for the contour plot when plotlypy_rendering=true is specified as a query parameter.

We will make this feature available to users (e.g., by preference panels) after we support the backend rendering of some other graphs.

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (9b2e894) 69.48% compared to head (aef91de) 69.38%.
Report is 30 commits behind head on main.

Files	Patch %	Lines
optuna_dashboard/_app.py	33.33%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #764      +/-   ##
==========================================
- Coverage   69.48%   69.38%   -0.11%     
==========================================
  Files          35       35              
  Lines        2353     2368      +15     
==========================================
+ Hits         1635     1643       +8     
- Misses        718      725       +7     

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[c-bata]

LGTM!