Fixing MRNN

[WenjieDu]

What does this PR do?

Fixing #283

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 7335631868

• 96 of 106 (90.57%) changed or added relevant lines in 5 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.04%) to 81.683%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/imputation/base.py	0	1	0.0%
pypots/imputation/mrnn/data.py	56	59	94.92%
pypots/cli/tuning.py	0	6	0.0%

Files with Coverage Reduction	New Missed Lines	%
pypots/cli/tuning.py	1	0.0%

Totals
Change from base Build 7328351922:	0.04%
Covered Lines:	5146
Relevant Lines:	6300

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💛 - Coveralls