Add type-hints to BaseLearner

[basnijholt]

Description

Please include a summary of the change and which (if so) issue is fixed.

Fixes #(ISSUE_NUMBER_HERE)

Checklist

• [ ] Fixed style issues using pre-commit run --all (first install using pip install pre-commit)
• [ ] pytest passed

Type of change

Check relevant option(s).

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] (Code) style fix or documentation update
• [ ] This change requires a documentation update