Add Informer, speed up CI testing, and make self-attention operator replaceable

[WenjieDu]

What does this PR do?

1. add Informer as an imputation model;
2. speed up CI testing；
3. make self-attention operator replaceable；
4. remove deprecated pypots.modules

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 8521696302

Details

• 228 of 277 (82.31%) changed or added relevant lines in 16 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.2%) to 80.819%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/nn/modules/transformer/attention.py	10	11	90.91%
pypots/nn/modules/transformer/layers.py	2	4	50.0%
pypots/imputation/informer/model.py	66	70	94.29%
pypots/imputation/informer/modules/submodules.py	102	144	70.83%

Files with Coverage Reduction	New Missed Lines	%
pypots/imputation/base.py	1	75.93%
pypots/nn/modules/transformer/layers.py	2	83.67%

Totals
Change from base Build 8511234308:	0.2%
Covered Lines:	6788
Relevant Lines:	8399

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💛 - Coveralls