Rename exec_aten:: to executorch::aten:: (#5296)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/5296

Modify any files that declare a `namespace exec_aten` to use the new `executorch::aten` namespace, and add an alias for the old namespace. Migration of users will happen in later diffs.

Reviewed By: mergennachin

Differential Revision: D62528437

fbshipit-source-id: 2ca7db5904dbc88cd97623abe0c0d67647fd0e5b