[ExecuTorch] Separate extension.Module Namespaces from Aten and non-Aten

Pull Request resolved: #10576

# Context

Separate `extension.Module` Namespaces to be `executorch::extension::module` and `executorch::extension::module::aten`, otherwise if a package relies on both aten and non-aten of the same implementation and the namespace is the same, there will be duplicate symbol issue like:

```
ld.lld: error: duplicate symbol: vtable for executorch::extension::Module
>>> defined at {redacted}/executorch/extension/module/__module__/__stripped__/module.cpp.pic.stripped.o:(vtable for executorch::extension::Module)
>>> defined at {redacted}/executorch/extension/module/__module_aten__/__stripped__/module.cpp.pic.stripped.o:
```

# Proposal

Doing something similar to what we already did for `bundled_program` in #10307

Since `extension.Module` is a public API, we introduce a namespace alias, so that existing use cases won't get affected. Since namespace alias doesn't create additional symbols, there won't be duplicate symbol issue.

ghstack-source-id: 281124468
@exported-using-ghexport

Differential Revision: [D73903870](https://our.internmc.facebook.com/intern/diff/D73903870/)

Author: zhenyan-zhang-meta

extension/module/module.h

@@ -16,8 +16,15 @@
 
 #include <executorch/runtime/executor/program.h>
 
+#ifdef USE_ATEN_LIB
+#define ET_MODULE_NAMESPACE module::aten
+#else // !USE_ATEN_LIB
+#define ET_MODULE_NAMESPACE module
+#endif // USE_ATEN_LIB
+
 namespace executorch {
 namespace extension {
+namespace ET_MODULE_NAMESPACE {
 
 using ET_RUNTIME_NAMESPACE::Method;
 using ET_RUNTIME_NAMESPACE::MethodMeta;
@@ -506,13 +513,21 @@ class Module {
   friend class ExecuTorchJni;
 };
 
+} // namespace ET_MODULE_NAMESPACE
 } // namespace extension
 } // namespace executorch
 
 namespace torch {
 namespace executor {
 // TODO(T197294990): Remove these deprecated aliases once all users have moved
 // to the new `::executorch` namespaces.
-using ::executorch::extension::Module;
+using ::executorch::extension::ET_MODULE_NAMESPACE::Module;
 } // namespace executor
 } // namespace torch
+
+namespace executorch {
+namespace extension {
+// backward compatible namespace alias
+using namespace ::executorch::extension::ET_MODULE_NAMESPACE;
+} // namespace extension
+} // namespace executorch