Refactor embedding package, remove the unused part in Autoformer, and update the docs

[WenjieDu]

What does this PR do?

1. put all embedding methods into pypots.nn.modules.transformer.embedding;
2. remove useless part in Autoformer;
3. update the docs;

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 8510512186

Details

• 33 of 36 (91.67%) changed or added relevant lines in 8 files are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.04%) to 80.607%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/nn/modules/transformer/embedding.py	24	27	88.89%

Files with Coverage Reduction	New Missed Lines	%
pypots/cli/env.py	4	57.14%

Totals
Change from base Build 8505967129:	-0.04%
Covered Lines:	6588
Relevant Lines:	8173

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💛 - Coveralls