Wrap Caffe2's SparseLengthsSum into a PyTorch op (#39596)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39596
This diff wraps Caffe2's SparseLengthsSum on GPU as a PT op.
Reviewed By: jianyuh
Differential Revision: D21895309
fbshipit-source-id: 38bb156f9be8d28225d2b44f5b4c93d27779aff9
