`local_weighted_learning.py`: fix `mypy` errors and more

[tianyizheng02]

Describe your change:

For local_weighted_learning.py:

• Fixed the mypy errors related to the file (that I accidentally introduced in #8069 💀), related to #8070
• Expand function documentation
• Add detailed explanation of algorithm at top of file
• Miscellaneous refactoring and renaming for clarity

• [ ] Add an algorithm?
• [x] Fix a bug or typo in an existing algorithm?
• [x] Documentation change?

Checklist:

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized.
• [x] I know that pull requests will not be merged if they fail the automated tests.
• [x] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• [ ] All new Python files are placed inside an existing directory.
• [ ] All filenames are in all lowercase characters with no spaces or dashes.
• [x] All functions and variable names follow Python naming conventions.
• [x] All function parameters and return values are annotated with Python type hints.
• [x] All functions have doctests that pass the automated testing.
• [x] All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• [x] If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[tianyizheng02]

@cclauss does this look good to merge?

[tianyizheng02]

@cclauss what do you think?

[tianyizheng02]

@ChrisO345 mind taking a look?