Add TEFN and implement it as an imputation model

[WenjieDu]

What does this PR do?

1. fixing Add TEFN #506;

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 10769470955

Details

• 129 of 132 (97.73%) changed or added relevant lines in 8 files are covered.
• 3 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.2%) to 83.273%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/imputation/tefn/model.py	61	64	95.31%

Files with Coverage Reduction	New Missed Lines	%
pypots/imputation/csdi/model.py	1	84.85%
pypots/classification/base.py	1	74.77%
pypots/clustering/vader/model.py	1	85.43%

Totals
Change from base Build 10695140728:	0.2%
Covered Lines:	11261
Relevant Lines:	13523

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💛 - Coveralls