Merge dev into main

[WenjieDu]

What does this PR do?

1. fixing Unify the API for all models to make predictions on the test set #198;
2. fixing GP-VAE met error when running on CUDA #200;
3. separating algorithms core and submodules;
4. enabling SAITS to use customized loss function;
5. making gene_physionet2012() to return ICUType;
6. updating 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.