Stop repeating input/output type info in docstrings

[leofang]

I think Sphinx is able to figure out the types and keep them in the document by inspecting the typing info, which going forward should be our source of truth (#312 (comment)). We should not repeat ourselves.

[leofang]

I noticed there are some issues for Sphinx to handle type alias properly. We need to proceed with caution.

sphinx-doc/sphinx#11991