Add Koopa modules and implement it as an imputation model

[WenjieDu]

What does this PR do?

1. fixing Add Koopa #377;

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 9050673798

Details

• 261 of 297 (87.88%) changed or added relevant lines in 8 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.1%) to 83.061%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/nn/modules/koopa/backbone.py	38	39	97.44%
pypots/imputation/koopa/model.py	71	74	95.95%
pypots/nn/modules/koopa/layers.py	114	146	78.08%

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

Totals
Change from base Build 9049850706:	0.1%
Covered Lines:	8900
Relevant Lines:	10715

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💛 - Coveralls