Implement Pyraformer as an imputation model

[WenjieDu]

What does this PR do?

1. fixing Add Pyraformer #378;

Before submitting

• 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 8980248383

Details

• 213 of 217 (98.16%) changed or added relevant lines in 8 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.3%) to 82.631%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/nn/modules/pyraformer/layers.py	77	78	98.72%
pypots/imputation/pyraformer/model.py	69	72	95.83%

Files with Coverage Reduction	New Missed Lines	%
pypots/imputation/csdi/model.py	1	84.62%
pypots/clustering/vader/model.py	1	85.28%

Totals
Change from base Build 8980243611:	0.3%
Covered Lines:	8192
Relevant Lines:	9914

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💛 - Coveralls