[c10] add operator= of intrusive_ptr to weak_intrusive_ptr (#44045)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/44045
Test Plan: Imported from OSS
Reviewed By: gmagogsfm
Differential Revision: D23632281
Pulled By: wanchaol
fbshipit-source-id: ea50427fc261f0c77ddaac2e73032827320d7077
