Close#941
rtfd build link:
https://readthedocs.org/projects/tianshou/builds/22019877/
Also -- fix two small issues reported by users, see #928 and #930
Note: I created the branch in thu-ml:tianshou instead of
Trinkle23897:tianshou to quickly check the rtfd build. It's not a good
process since every commit would trigger twice CI pipelines :(
