add wandb hook and use it in tutorial 00

[tztsai]

Description

I added some changes to the "hooks" folder, including adding a Hook class, renaming DefaultHooks to DefaultHook, and adding WandbHook. The Hooks object now becomes a container of Hooks, initialized by Hooks([<Hook>, ...]), and adding a hook can be easily done by <Hooks>.append(<Hook>).

How has this been tested?

• I tested it by adding a WandbHook in the hooks of a classification study in Tutorial 0. The metrics were successfully logged in my wandb project.

Checklist

• I have followed the Contribution Guidelines and Code of Conduct
• I have commented my code following the van der Schaar Lab Styleguide
• I have labelled this PR with the relevant Type labels
• My changes are covered by tests