text: Title: decorated functions are not properly typechecked
  Body: <!-- Describe the bug report / feature request here -->
  We will not catch typecheck errors on functions with have e.g. `@timeline.event` or `@usage_lib.entrypoint`.
  
  See https://mypy.readthedocs.io/en/stable/generics.html#declaring-decorators
  
  <!-- If relevant, fill in versioning info to help us troubleshoot -->
  _Version & Commit info:_
  * `sky -v`: PLEASE_FILL_IN
  * `sky -c`: PLEASE_FILL_IN
  