Remove python_default_init from ATen and use Optional (#15234)
Summary:
Optional clean up. This PR remove python_default_init from the yaml files, and the code-gen, and utilize optional type to do the work.
This also fix the bug in the #13149 to correctly adopt as_strided backward.
Fixes #9941
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15234
Differential Revision: D13502044
Pulled By: wanchaol
fbshipit-source-id: 774b61fc4414482cf11d56e22bd0275aefb352a4
