[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/frontend/function_schema_parser.cpp

@@ -23,8 +23,7 @@ using c10::make_right;
 using c10::OperatorName;
 using c10::OptionalType;
 
-namespace torch {
-namespace jit {
+namespace torch::jit {
 
 namespace {
 struct SchemaParser {
@@ -390,5 +389,4 @@ OperatorName parseName(const std::string& name) {
   return std::move(parsed.left());
 }
 
-} // namespace jit
-} // namespace torch
+} // namespace torch::jit