add half dtype to slice_channel codegen (#4025)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4025

slice_batch_height_width supports half but not slice_channel. Adding half dtype to the yaml + updating to use new layout declaration codegen method

Reviewed By: jorgep31415

Differential Revision: D58767453

fbshipit-source-id: a83af6d59849dd295d8969dc253fb08a18d1d68a