Refactor code and update docstring

[WenjieDu]

What does this PR do?

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).
• 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 8965153787

Details

• 13 of 45 (28.89%) changed or added relevant lines in 25 files are covered.
• 10 unchanged lines in 6 files lost coverage.
• Overall coverage increased (+0.1%) to 81.801%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/classification/brits/model.py	0	1	0.0%
pypots/clustering/crli/model.py	0	1	0.0%
pypots/clustering/vader/model.py	0	1	0.0%
pypots/imputation/gpvae/model.py	0	1	0.0%
pypots/forecasting/bttf/model.py	0	2	0.0%
pypots/forecasting/csdi/model.py	0	2	0.0%
pypots/imputation/autoformer/model.py	0	2	0.0%
pypots/imputation/crossformer/model.py	0	2	0.0%
pypots/imputation/csdi/model.py	0	2	0.0%
pypots/imputation/dlinear/model.py	0	2	0.0%

Files with Coverage Reduction	New Missed Lines	%
pypots/classification/brits/model.py	1	96.77%
pypots/imputation/gpvae/model.py	1	86.01%
pypots/imputation/base.py	2	77.06%
pypots/classification/base.py	2	74.31%
pypots/clustering/base.py	2	37.62%
pypots/forecasting/base.py	2	35.78%

Totals
Change from base Build 8964194855:	0.1%
Covered Lines:	7776
Relevant Lines:	9506

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💛 - Coveralls