Apply SAITS loss to newly added models and update the docs

[WenjieDu]

What does this PR do?

1. apply SAITS loss to Crossformer, PatchTST, DLinear, ETSformer, FEDformer, Informer, and Autoformer;
2. 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).
• 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 8644947117

Details

• 56 of 56 (100.0%) changed or added relevant lines in 16 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 81.153%

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Totals
Change from base Build 8616441468:	0.1%
Covered Lines:	6885
Relevant Lines:	8484

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💛 - Coveralls