Add ETSformer as an imputation model

[WenjieDu]

What does this PR do?

1. add ETSformer 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 8488788422

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

• 333 of 344 (96.8%) changed or added relevant lines in 7 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.7%) to 82.867%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/imputation/etsformer/model.py	67	71	94.37%
pypots/imputation/etsformer/modules/submodules.py	230	237	97.05%

Totals
Change from base Build 8488533632:	0.7%
Covered Lines:	6065
Relevant Lines:	7319

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💛 - Coveralls