[PyTorch] Add & document borrow_from_optional_tensor (#56647)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56647
This should be more efficient than the old hacky wrapper for optional Tensor pattern. Despite appearances, the old pattern did a reference count bump for non-empty optionals. Following diff will contain an automated change to migrate callsites.
ghstack-source-id: 127112926
Test Plan: Review, CI on following change
Reviewed By: bhosmer
Differential Revision: D27925838
fbshipit-source-id: 2c6082c5930b1e71b853a75c52873088dbc48167
