Half support: unary ops with realh pattern (#1709)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1709

Updated the following 2 ops to support Half dtype:
- ceil
- floor
ghstack-source-id: 213131133
exported-using-ghexport

Reviewed By: mikekgfb

Differential Revision: D53074613

fbshipit-source-id: f90c263f5d91a1ea6df8964b9c96dc4e05c9ee48