Implement iTransformer as an imputation model

[WenjieDu]

What does this PR do?

1. fixing Add iTransformer #366

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 8877547978

Details

• 119 of 123 (96.75%) changed or added relevant lines in 5 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.2%) to 81.872%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/imputation/itransformer/model.py	75	79	94.94%

Files with Coverage Reduction	New Missed Lines	%
pypots/clustering/vader/model.py	1	85.86%

Totals
Change from base Build 8877407725:	0.2%
Covered Lines:	7845
Relevant Lines:	9582

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💛 - Coveralls