Make TripletMarginLossImpl subclass from Cloneable (#27956)
Summary:
Continuing from https://github.com/pytorch/pytorch/pull/27770 to make all `torch::nn` layers subclass from `torch::nn::Cloneable`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27956
Differential Revision: D17936555
Pulled By: yf225
fbshipit-source-id: 75f7982e7893675cf6da0f5419224b92af579818
