s/exec_aten::/executorch::aten::/ for extension/**/*.h (#6032)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/6032

Migrate all extension headers to use the new aten namespace, so that they act as good examples for users. The .cpp code can migrate later.

Reviewed By: lucylq

Differential Revision: D64079593

fbshipit-source-id: 62164cd6ec3238e21e896813f185d0c1446ef527