Removing deprecated save_model and load_model, adding the imputation model Autoformer

[WenjieDu]

What does this PR do?

1. permanently remove deprecated functions save_model() and load_model();
2. add the imputation model Autoformer;

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 8448797394

Details

• 276 of 341 (80.94%) changed or added relevant lines in 8 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 81.788%

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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 8340518079:	0.03%
Covered Lines:	5479
Relevant Lines:	6699

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💛 - Coveralls