[nnc] Fix half2float conversion and re-enable float16 (#64199)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/64199
Test Plan: Imported from OSS
Reviewed By: ZolotukhinM
Differential Revision: D30643865
Pulled By: bertmaher
fbshipit-source-id: 9de6adca53bd08839328cbaf6364f7de9550264b
