Fix no global destructor for strings in training modules (#12521)

### Summary

When building with
[-Wglobal-constructors](https://clang.llvm.org/docs/DiagnosticsReference.html#wglobal-constructors),
the non-static strings throw an error since they don't have a global
destructor. This diff fixes that.

### Test plan

CI + internal build

Author: jathu

extension/training/module/training_module.cpp

@@ -13,9 +13,19 @@ namespace extension {
 namespace training {
 
 namespace {
-std::string gradients_method_prefix = "__et_training_gradients_index_";
-std::string parameters_method_prefix = "__et_training_parameters_index_";
-std::string fqn_method_prefix = "__et_training_fqn_";
+
+std::string make_parameters_method_name(const std::string& method_name) {
+  return "__et_training_parameters_index_" + method_name;
+}
+
+std::string make_gradients_method_name(const std::string& method_name) {
+  return "__et_training_gradients_index_" + method_name;
+}
+
+std::string make_fqn_method_name(const std::string& method_name) {
+  return "__et_training_fqn_" + method_name;
+}
+
 } // namespace
 
 runtime::Result<std::vector<runtime::EValue>>
@@ -24,15 +34,15 @@ TrainingModule::execute_forward_backward(
     const std::vector<runtime::EValue>& input) {
   // Find where the user outputs end.
   const std::string gradients_method_name =
-      gradients_method_prefix + method_name;
+      make_gradients_method_name(method_name);
   auto res = executorch::extension::Module::execute(gradients_method_name);
   if (!res.ok()) {
     return res.error();
   }
   uint64_t grad_start = res.get()[0].toInt();
 
   const std::string parameters_method_name =
-      parameters_method_prefix + method_name;
+      make_parameters_method_name(method_name);
   // get params start.
   auto param_res =
       executorch::extension::Module::execute(parameters_method_name);
@@ -66,7 +76,7 @@ TrainingModule::execute_forward_backward(
     auto& gradients_map = method_named_gradients_.at(method_name);
 
     // Get names if we havent seen this method before.
-    const std::string fqn_method_name = fqn_method_prefix + method_name;
+    const std::string fqn_method_name = make_fqn_method_name(method_name);
     auto fqn_res = executorch::extension::Module::execute(fqn_method_name);
     if (!fqn_res.ok()) {
       return fqn_res.error();
@@ -92,9 +102,9 @@ TrainingModule::named_parameters(const std::string& method_name) {
   // If we haven't seen this method before, populate the dict.
   if (method_named_parameters_.find(method_name) ==
       method_named_parameters_.end()) {
-    const std::string fqn_method_name = fqn_method_prefix + method_name;
+    const std::string fqn_method_name = make_fqn_method_name(method_name);
     const std::string parameters_method_name =
-        parameters_method_prefix + method_name;
+        make_parameters_method_name(method_name);
 
     method_named_parameters_.insert({method_name, {}});