Add GRU-D as an imputation model

[WenjieDu]

What does this PR do?

1. fixing Implement GRU-D as an imputation model #416;

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 9125845957

Details

• 131 of 134 (97.76%) changed or added relevant lines in 6 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.2%) to 84.029%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/imputation/grud/data.py	41	44	93.18%

Files with Coverage Reduction	New Missed Lines	%
pypots/forecasting/csdi/model.py	1	83.82%

Totals
Change from base Build 9124698085:	0.2%
Covered Lines:	9665
Relevant Lines:	11502

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💛 - Coveralls