Add models US-GAN and GP-VAE, update docs, refactor testing cases, add cal_internal_cluster_validation_metrics()

[WenjieDu]

What does this PR do?

1. fixing Add GP-VAE/USGAN #175, adding US-GAN and GP-VAE;
2. update docs;
3. refactor testing cases;
4. add pypots.utils.metrics.cal_internal_cluster_validation_metrics();

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 6263888182

• 0 of 492 (0.0%) changed or added relevant lines in 17 files are covered.
• 2976 unchanged lines in 72 files lost coverage.
• Overall coverage decreased (-82.8%) to 0.0%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/base.py	0	1	0.0%
pypots/classification/grud/data.py	0	1	0.0%
pypots/imputation/brits/data.py	0	1	0.0%
pypots/imputation/mrnn/module.py	0	1	0.0%
pypots/data/saving.py	0	2	0.0%
pypots/imputation/gpvae/init.py	0	2	0.0%
pypots/imputation/saits/data.py	0	2	0.0%
pypots/imputation/usgan/init.py	0	2	0.0%
pypots/imputation/init.py	0	3	0.0%
pypots/data/base.py	0	4	0.0%

Files with Coverage Reduction	New Missed Lines	%
pypots/utils/init.py	1	0.0%
pypots/classification/brits/init.py	2	0.0%
pypots/classification/grud/init.py	2	0.0%
pypots/classification/raindrop/init.py	2	0.0%
pypots/clustering/crli/init.py	2	0.0%
pypots/clustering/vader/data.py	2	0.0%
pypots/clustering/vader/init.py	2	0.0%
pypots/data/load_preprocessing.py	2	0.0%
pypots/forecasting/bttf/init.py	2	0.0%
pypots/forecasting/init.py	2	0.0%

Totals
Change from base Build 6221803488:	-82.8%
Covered Lines:	0
Relevant Lines:	4107

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💛 - Coveralls