[BE] Nested namespace in `ATen/native` headers (#103753)
Use nested namespace and `enum class` in `ATen/native` headers.
In particular, it helps avoid polluting global namespace with `MAX`,`MIN` enum values.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/103753
Approved by: https://github.com/atalman, https://github.com/Skylion007
