Smooth L1 loss (#27661)
Summary:
In accordance with https://github.com/pytorch/pytorch/issues/25883, I added the `SmoothL1Loss` module and `smooth_l1_loss` functional.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27661
Differential Revision: D18002332
Pulled By: yf225
fbshipit-source-id: b382df8becb0de14986ec16ee0dc953d7b10e917
