Add Crossformer, Autoformer, PatchTST, DLinear, ETSformer, FEDformer as imputation models

[WenjieDu]

What does this PR do?

1. add Crossformer as an imputation method;
2. add Autoformer as an imputation method;
3. add PatchTST as an imputation method;
4. add DLinear as an imputation method;
5. add ETSformer as an imputation method;
6. add FEDformer as an imputation method;

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 8505334559

Details

• 986 of 1345 (73.31%) changed or added relevant lines in 26 files are covered.
• 3 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-1.4%) to 80.66%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/imputation/fedformer/modules/core.py	25	28	89.29%
pypots/imputation/crossformer/model.py	68	72	94.44%
pypots/imputation/dlinear/model.py	62	66	93.94%
pypots/imputation/etsformer/model.py	67	71	94.37%
pypots/imputation/fedformer/model.py	69	73	94.52%
pypots/imputation/etsformer/modules/submodules.py	230	237	97.05%
pypots/imputation/dlinear/modules/core.py	32	44	72.73%
pypots/imputation/crossformer/modules/submodules.py	90	127	70.87%
pypots/imputation/fedformer/modules/submodules.py	225	509	44.2%

Files with Coverage Reduction	New Missed Lines	%
pypots/imputation/csdi/model.py	1	83.72%
pypots/cli/env.py	1	65.31%
pypots/imputation/usgan/model.py	1	88.0%

Totals
Change from base Build 8477330697:	-1.4%
Covered Lines:	6602
Relevant Lines:	8185

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💛 - Coveralls