[torchgen] Fix selective build error on custom namespace (#83141)
Summary: Currently `SelectiveBuilder` is hardcoding namespace `aten` for operators. This is not working anymore since operators started to have custom namespaces. This fixes it.
Test Plan: Rely on newly added unit test
Differential Revision: D38565527
Pull Request resolved: https://github.com/pytorch/pytorch/pull/83141
Approved by: https://github.com/JacobSzwejbka
