text: Title: Help add mypy Type Annotations to all code files
  Body: Contributors: we could really use your help with this effort! Adding type annotations (that pass mypy with `--strict` flag) to every function in the codebase is a lot of work, but it will make our code much more robust and help avoid future bugs.
  
  Just start with any one file and help us add type annotations (that pass mypy with `--strict` flag) for the functions in this file. You can make a PR that only covers one file which shouldn't take that long.
  
  To get started, just run:
  ```
  mypy --strict --install-types --non-interactive cleanlab
  ```
  
  and see which files it complains about. You can add strict type annotations for any one these files. After you have saved some changes, you can run the above command again to see if mypy is now happy with the strict type annotations you have added.
  
  
  Also check out: 
  https://github.com/cleanlab/cleanlab/issues/307
  https://github.com/cleanlab/cleanlab/issues/351
  
  Example PR that added type annotations for one file:  https://github.com/cleanlab/cleanlab/pull/317
  You can similarly copy this pattern for other files.
  