Enable hyperparameter tuning with NNI, fix dependency error in testing_CI, and update docs

[WenjieDu]

What does this PR do?

1. fixing Hyperparameter optimization #187;
2. fixing Workflow testing_CI failed because of dependency error #222;
3. updating docs;

Before submitting

• This PR is made to fix a typo or improve the docs (you can dismiss the other checks if this is the case).
• Was this discussed/approved via a GitHub issue? Please add a link to it if that's the case.
• I have commented my code, particularly in hard-to-understand areas.
• I have written necessary tests and already run them locally.

[coveralls]

Pull Request Test Coverage Report for Build 6703021294

• 38 of 110 (34.55%) changed or added relevant lines in 12 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.0%) to 84.185%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/cli/pypots_cli.py	0	2	0.0%
pypots/classification/base.py	7	12	58.33%
pypots/clustering/crli/model.py	7	12	58.33%
pypots/clustering/vader/model.py	7	12	58.33%
pypots/clustering/base.py	5	12	41.67%
pypots/forecasting/base.py	5	12	41.67%
pypots/cli/tuning.py	0	41	0.0%

Totals
Change from base Build 6559214085:	-1.0%
Covered Lines:	4381
Relevant Lines:	5204

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💛 - Coveralls