Implement FreTS as an imputation model

[WenjieDu]

What does this PR do?

1. fixing Add FreTS #368;

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 8877251561

Details

• 156 of 166 (93.98%) changed or added relevant lines in 7 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.2%) to 81.689%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/imputation/frets/model.py	64	67	95.52%
pypots/nn/modules/frets/backbone.py	49	56	87.5%

Files with Coverage Reduction	New Missed Lines	%
pypots/imputation/gpvae/model.py	1	86.81%

Totals
Change from base Build 8877231764:	0.2%
Covered Lines:	7727
Relevant Lines:	9459

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💛 - Coveralls