[BE] Use nested namespaces in .cpp/.cu files (#92100)

As we live in C++17 world

This is a functional no-op, just
- `s/namespace at { namespace native {/namespace at::native {/`
- `s/namespace torch { namespace jit {/namespace torch::jit {/`

Pull Request resolved: https://github.com/pytorch/pytorch/pull/92100
Approved by: https://github.com/izaitsevfb

torch/csrc/jit/python/pybind_utils.cpp

@@ -12,8 +12,7 @@
 
 #include <limits>
 
-namespace torch {
-namespace jit {
+namespace torch::jit {
 
 static thread_local bool allow_numbers_as_tensors = false;
 
@@ -789,5 +788,4 @@ py::object _get_operation_for_overload_or_packet(
   return invokeOperatorFromPython(operations, args, kwargs, dk);
 }
 
-} // namespace jit
-} // namespace torch
+} // namespace torch::jit