some documentation and style fixes to smooth_l1_loss (#45587)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/45587
Test Plan: Imported from OSS
Reviewed By: mrshenli
Differential Revision: D24024313
Pulled By: bdhirsh
fbshipit-source-id: c50efb2934d7b9d3b090e92678319cde42c0df45
