Simplifying the structure of val_set, and using a consistent strategy when lazy-loading val_set

[WenjieDu]

What does this PR do?

1. fixing Simplify the structure of val_set #271;
2. fixing Make the framework using a consistent strategy when lazy-loading val_set #270;

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 7243404330

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• 263 of 319 (82.45%) changed or added relevant lines in 31 files are covered.
• 7 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.5%) to 82.933%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/data/checking.py	9	10	90.0%
pypots/imputation/brits/model.py	8	9	88.89%
pypots/imputation/csdi/model.py	9	10	90.0%
pypots/imputation/gpvae/model.py	8	9	88.89%
pypots/imputation/mrnn/model.py	8	9	88.89%
pypots/imputation/saits/model.py	7	8	87.5%
pypots/imputation/timesnet/model.py	7	8	87.5%
pypots/imputation/transformer/model.py	7	8	87.5%
pypots/imputation/usgan/model.py	8	9	88.89%
pypots/data/utils.py	6	9	66.67%

Files with Coverage Reduction	New Missed Lines	%
pypots/classification/base.py	1	74.07%
pypots/classification/brits/model.py	3	95.31%
pypots/data/saving/h5.py	3	81.48%

Totals
Change from base Build 7219615511:	0.5%
Covered Lines:	4937
Relevant Lines:	5953

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💛 - Coveralls