[Doc] Add typing hints / mypy types cleanup (#3816)

Co-authored-by: Roger Wang <136131678+ywang96@users.noreply.github.com>

docs/source/conf.py

@@ -12,6 +12,7 @@
 
 import logging
 import sys
+from typing import List
 
 from sphinx.ext import autodoc
 
@@ -45,7 +46,7 @@ templates_path = ['_templates']
 # List of patterns, relative to source directory, that match files and
 # directories to ignore when looking for source files.
 # This pattern also affects html_static_path and html_extra_path.
-exclude_patterns = []
+exclude_patterns: List[str] = []
 
 # Exclude the prompt "$" when copying code
 copybutton_prompt_text = r"\$ "