add models GP-VAE/USGAN

[AugustJW]

What does this PR do?

Fixing #175

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 5927686933

• 143 of 649 (22.03%) changed or added relevant lines in 10 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-9.0%) to 74.337%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/base.py	0	1	0.0%
pypots/imputation/gpvae/data.py	10	30	33.33%
pypots/imputation/usgan/data.py	10	35	28.57%
pypots/imputation/usgan/modules.py	17	47	36.17%
pypots/imputation/gpvae/modules.py	24	125	19.2%
pypots/imputation/gpvae/model.py	29	152	19.08%
pypots/imputation/usgan/model.py	44	250	17.6%

Files with Coverage Reduction	New Missed Lines	%
pypots/forecasting/base.py	1	34.09%

Totals
Change from base Build 5916043036:	-9.0%
Covered Lines:	3253
Relevant Lines:	4376

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💛 - Coveralls

[github-actions]

This pull request had no activity for 14 days. It will be closed in 1 week unless there is some new activity.

[WenjieDu]

Hey Jun, thanks for your PR! Although this is your first PR on GitHub, I can see you're really trying to contribute something to PyPOTS and make it better. Nice job!

I'm going to validate and refactor your code to make us able to merge it.