Add Autoformer as an imputation model

[WenjieDu]

What does this PR do?

1. as New Model #295 requested, add Autoformer 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 8448510509

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 276 of 341 (80.94%) changed or added relevant lines in 8 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.04%) to 81.773%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/imputation/autoformer/model.py	67	71	94.37%
pypots/imputation/autoformer/modules/submodules.py	171	232	73.71%

Totals
Change from base Build 8415549782:	-0.04%
Covered Lines:	5478
Relevant Lines:	6699

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💛 - Coveralls