Add Half dtype support (#1795)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1795

Rationale - this was needed to bypass errors while enabling xnnpack fp16 support up the stack.

* Add Half dtype for Aten bridge
* Allow serialization of Half - potentially FC breaking (already have it in [scalar types](https://www.internalfb.com/code/fbsource/[5dd332f406a1be2e184a14f83815a7bfb3a65ccd]/xplat/executorch/runtime/core/portable_type/scalar_type.h?lines=67))

Reviewed By: larryliu0820

Differential Revision: D53248905

fbshipit-source-id: 65c35c03c7ee17e949c05591d945e24e2d003f67