Simplifying val_set, renaming X_intact, and adding unit tests for the visual package

[WenjieDu]

What does this PR do?

1. fixing Simplify the structure of val_set #271;
2. fixing Make the framework using a consistent strategy when lazy-loading val_set #270;
3. fixing The name of X_intact may be confusing #273;
4. writing unit testing cases for visualization functions and adding matplotlib as a dependency;

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.