Add stack & cat support for CPU Half (#16389)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/6968
Needed for #14705
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16389
Differential Revision: D13861446
Pulled By: gchanan
fbshipit-source-id: 7b8700b95aaf252d9669693dbddccb2302e58409
