Adding CSDI, updating the docs

[WenjieDu]

What does this PR do?

1. fixing Add the model CSDI  #207;
2. updating 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 6484195816

• 339 of 383 (88.51%) changed or added relevant lines in 12 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.2%) to 85.233%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/imputation/csdi/modules/submodules.py	105	107	98.13%
pypots/imputation/csdi/model.py	56	59	94.92%
pypots/imputation/csdi/data.py	26	41	63.41%
pypots/imputation/csdi/modules/core.py	123	147	83.67%

Files with Coverage Reduction	New Missed Lines	%
pypots/modules/self_attention.py	1	73.61%

Totals
Change from base Build 6459962947:	0.2%
Covered Lines:	4398
Relevant Lines:	5160

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💛 - Coveralls