Add FITS

[WenjieDu]

What does this PR do?

1. fixing Add FITS #519;

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 11010266015

Details

• 133 of 144 (92.36%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 83.443%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/imputation/fits/core.py	30	32	93.75%
pypots/imputation/fits/model.py	64	67	95.52%
pypots/nn/modules/fits/backbone.py	26	32	81.25%

Totals
Change from base Build 10981319450:	0.1%
Covered Lines:	11329
Relevant Lines:	13577

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💛 - Coveralls