Move skorch

[eukaryo]

Motivation

Progress optuna/optuna#4484

Description of the changes

• Move integration/skorch.py and its test from optuna/optuna.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (main@ca3833f). Click here to learn what that means.
The diff coverage is n/a.

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@@          Coverage Diff           @@
##             main     #22   +/-   ##
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  Files           ?      12           
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[HideakiImamura]

@eukaryo Please resolve conflicts.

[eukaryo]

@HideakiImamura I have resolved current conflict, and all checks have passed.
If other conflicts occur, I am fine with that. So you may prioritize the merging of another pull request. I will follow your decision.

[HideakiImamura]

LGTM.