Code refactor

[WenjieDu]

What does this PR do?

Doing some code refactoring in this PR.

Before submitting

• 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 7061635412

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A quick fix for this PR: rebase it; your next report should be accurate.

• 274 of 320 (85.63%) changed or added relevant lines in 21 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.05%) to 83.424%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/modules/transformer/pos_enc.py	19	20	95.0%
pypots/imputation/csdi/modules/core.py	7	9	77.78%
pypots/imputation/gpvae/modules/core.py	24	29	82.76%
pypots/modules/transformer/layers.py	36	44	81.82%
pypots/modules/transformer/auto_encoder.py	14	44	31.82%

Totals
Change from base Build 7028983043:	0.05%
Covered Lines:	4469
Relevant Lines:	5357

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💛 - Coveralls