Clean up non-exec_aten references to tensor types (#5254)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/5254

Most code should use the exec_aten:: types: only aten-mode-aware code should refer directly to e.g. torch::executor::Tensor.

ArrayRef is a little different because it's also a core type, but in that case it should be called executorch::runtime::ArrayRef.

Reviewed By: mergennachin

Differential Revision: D62476845

fbshipit-source-id: 869fb15ce342873697823425271955ca7ed4c14d