Modularize NN models, remove deprecated metric funcs, and update docs

[WenjieDu]

What does this PR do?

1. modularize NN models Modularize neural-net models to increase code reuse rate #347, Multitask models should be across tasks #242;
2. remove deprecated metric funcs;
3. reduce the testing dataset size for speeding up CI;
4. 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.