Add StemGNN modules and implement it as an imputation model

[WenjieDu]

What does this PR do?

1. fixing Add StemGNN #413;

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 9124199219

Details

• 242 of 253 (95.65%) changed or added relevant lines in 8 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.3%) to 83.861%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/imputation/stemgnn/model.py	66	69	95.65%
pypots/nn/modules/stemgnn/backbone.py	72	80	90.0%

Files with Coverage Reduction	New Missed Lines	%
pypots/imputation/csdi/model.py	1	83.33%
pypots/clustering/vader/model.py	1	84.42%

Totals
Change from base Build 9064214275:	0.3%
Covered Lines:	9535
Relevant Lines:	11370

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💛 - Coveralls