Add Crossformer as an imputation model

[WenjieDu]

What does this PR do?

1. add Crossformer as an imputation model, as suggested in New Model #295;

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 8489473963

Details

• 205 of 246 (83.33%) changed or added relevant lines in 7 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.03%) to 82.882%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/imputation/crossformer/model.py	68	72	94.44%
pypots/imputation/crossformer/modules/submodules.py	90	127	70.87%

Files with Coverage Reduction	New Missed Lines	%
pypots/cli/env.py	2	61.22%

Totals
Change from base Build 8488918250:	0.03%
Covered Lines:	6270
Relevant Lines:	7565

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💛 - Coveralls