Soft Margin loss (#27660)
Summary:
In accordance with https://github.com/pytorch/pytorch/issues/25883, I added the `SoftMarginLoss` module and `soft_margin_loss` functional.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27660
Differential Revision: D17958325
Pulled By: yf225
fbshipit-source-id: c14422765e6e1fdabf6c9687080e6d5ff490d300
