Modularize neural network models

[WenjieDu]

What does this PR do?

1. fixing Modularize neural-net models to increase code reuse rate #347;
2. fixing Multitask models should be across tasks #242;

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 8671647610

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 1021 of 1150 (88.78%) changed or added relevant lines in 91 files are covered.
• 4 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.06%) to 81.089%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/clustering/vader/model.py	2	3	66.67%
pypots/imputation/saits/core.py	38	39	97.44%
pypots/imputation/timesnet/core.py	10	11	90.91%
pypots/nn/modules/csdi/backbone.py	11	12	91.67%
pypots/nn/modules/autoformer/auto_encoder.py	7	11	63.64%
pypots/nn/modules/gpvae/backbone.py	32	39	82.05%
pypots/classification/raindrop/core.py	33	46	71.74%
pypots/nn/modules/crossformer/auto_encoder.py	17	31	54.84%
pypots/nn/modules/fedformer/autoencoder.py	17	31	54.84%
pypots/nn/modules/informer/layers.py	3	20	15.0%

Files with Coverage Reduction	New Missed Lines	%
pypots/clustering/vader/model.py	1	84.85%
pypots/imputation/gpvae/model.py	1	85.42%
pypots/cli/env.py	2	57.14%

Totals
Change from base Build 8644947117:	-0.06%
Covered Lines:	7105
Relevant Lines:	8762

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💛 - Coveralls