Code fix/download csv

[eukaryo]

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

I will take over this PR:
#648

What does this implement/fix? Explain your changes.

• added a function to make a csv file in backend.
• added a button to download the file in frontend. (in the bottom of TrialTable page)

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (9e314a9) 67.20% compared to head (f63b5e9) 67.66%.
Report is 24 commits behind head on main.

Files	Patch %	Lines
optuna_dashboard/_app.py	94.59%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #723      +/-   ##
==========================================
+ Coverage   67.20%   67.66%   +0.46%     
==========================================
  Files          35       35              
  Lines        2293     2329      +36     
==========================================
+ Hits         1541     1576      +35     
- Misses        752      753       +1     

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[eukaryo]

The download button appears like this:

[c-bata]

@nabenabe0928 @HideakiImamura Could you review this PR?

[nabenabe0928]

Thank you for the PR, the new feature looks very convenient!
I checked the Python file for now and refactored a bit.

[HideakiImamura]

I think you should check the all of studies in https://gist.github.com/HideakiImamura/668d6f5703c58255e54ece03cd270f2f.

[eukaryo]

I tried moving the button at the top, and making the text Download CSV file itself also as a part of button.

[HideakiImamura]

LGTM.

[nabenabe0928]

Thank you for the changes, LGTM!!
I added simple tests, please check below.

[nabenabe0928]

Sorry, I added simple tests.

[eukaryo]

Thank you for adding the tests! I suppose there was another option to separate such expansions into a different PR, but for this instance, I think it's fine to include them in this PR.

[nabenabe0928]

Thank you for adding the tests, LGTM!