Refactor add_tags method

[gabegma]

Part of #297

Description:

• Refactor add_tags method so it is cleaner. it seems some code was redundant and that we were saving prediction tags in the base dataset split.
• This triggered many mypy errors so I needed to refactor further the types related to tags.
• This is preliminary work before importing proposed actions.

Checklist:

You should check all boxes before the PR is ready. If a box does not apply, check it to acknowledge it.

• ISSUE NUMBER. You linked the issue number (Ex: Resolve #XXX).
• PRE-COMMIT. You ran pre-commit on all commits, or else, you
  ran pre-commit run --all-files at the end.
• USER CHANGES. The changes are added to CHANGELOG.md and the documentation, if they impact
  our users.
• DEV CHANGES.
  • Update the documentation if this PR changes how to develop/launch on the app.
  • Update the README files and our wiki for any big design decisions, if relevant.
  • Add unit tests, docstrings, typing and comments for complex sections.

[JosephMarinier]

Wow! 😲 Good job! 👌 Thank you for cleaning that up.