[Codegen] Expose namespace argument for static dispatch (#77710)
For static dispatch we are hardcoding namespace to be `at` for backend-specific C++ functions, e.g., `at::cpu::add()`. We are extending it to accept namespaces from callsite. This is a temporary solution, in the long run we want to introduce custom namespace into codegen system, e.g., we should be able to add `at::` to `native_functions.yaml` and parse it into `NativeFunction`. This needs a bit more design.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/77710
Approved by: https://github.com/ezyang
