[caffe2] Add operator schema for FP16SparseNorm (#46300)
Summary:
Fixes regression introduced by https://github.com/pytorch/pytorch/pull/45551
Also Fix signed-unsigned comparison warnings in test/cpp/tensorexpr/test_train_impl.cpp
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46300
Reviewed By: walterddr
Differential Revision: D24294821
Pulled By: malfet
fbshipit-source-id: 16bffa71ec0d2d38208855223a3c5efb18414ab5
