Use c10::variant-based enums for SmoothL1Loss module and functional
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/29536
Test Plan: Imported from OSS
Differential Revision: D18432272
Pulled By: yf225
fbshipit-source-id: fa355145962e93025b7de98b99b0a4fc82e8c871
