Refactor algorithms' module structure, enable customized loss function in SAITS, enable GP-VAE to run on CUDA, etc.

[WenjieDu]

What does this PR do?

1. separate algorithms core and submodules;
2. enable SAITS to use customized loss function, related to the issue Enabling users to customize loss function #137;
3. make gene_physionet2012() to return ICUType;
4. fixing GP-VAE met error when running on CUDA #200;
5. update 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.