Support DefaultBackend keyword in native_functions.yaml. (#45719)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/45719
Test Plan: Imported from OSS
Reviewed By: bhosmer
Differential Revision: D24165888
Pulled By: ailzhang
fbshipit-source-id: 9b3c5e71f5b6a985e1a43157813e7d77dbe13b07
