MaybeOwned page for dev wiki (#63450)

MaybeOwned page for dev wiki (#63450)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63450

Brief guide to understanding `MaybeOwned<Tensor>`, aimed at C++ PT devs who are obliged to interact with existing uses of it, rather than encouraging new usage.

For reviewers: I haven't yet added a link to this page from anywhere. I'm thinking the right place is the [dev wiki main page C++ section](https://github.com/pytorch/pytorch/wiki#c) but happy to put it wherever makes sense, suggestions welcome.

Test Plan: Imported from OSS

Reviewed By: navahgar

Differential Revision: D30402313

Pulled By: bhosmer

fbshipit-source-id: 69b15909ecafcd8d88e44f664f88c3ad4eb26d84