rm sk-learn's GridSearchCV's iid option in its init function

[DawnweiSong]

Pull Request Template

iid option of GridSearchCV was removed since sk-learn 0.24 (reference: https://github.com/scikit-learn/scikit-learn/blob/d38d1cbd72a6f001ed58d261b71bd99eb6ff2232/doc/whats_new/v0.20.rst#L1220) . However, sk-dist still kept this old option, which will raise error in initialization. I have removed this param to be compatible with newer sk-learn.

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Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [X] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ ] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [X] I have read the CONTRIBUTING document.
• [ ] I have added reviewers to the PR.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.