Implement Non-stationary Transformer as an imputation model

[WenjieDu]

What does this PR do?

1. fixing Add Non-stationary Transformer #376;

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 8980157157

Details

• 188 of 192 (97.92%) changed or added relevant lines in 9 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.5%) to 82.283%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/imputation/nonstationary_transformer/model.py	70	74	94.59%

Totals
Change from base Build 8969694337:	0.5%
Covered Lines:	7979
Relevant Lines:	9697

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💛 - Coveralls