message: Simplify `SavedModelSignatureDefImporterLite::Convert()`
  1. Remove duplicate invocations.
  2. Make `Convert()` a naive pass-through method to reduce the mental stack depth.
diff: @@ -3503,15 +3503,9 @@ class SavedModelSignatureDefImporterLite {
  static StatusOr<mlir::OwningModuleRef> Convert(
  SavedModelMLIRImportInput& input, absl::Span<std::string> exported_names,
  mlir::MLIRContext* context, bool import_restore = true) {
  - LoadImporterDialects(*context);
  SavedModelSignatureDefImporterLite importer(input, exported_names, context,
  import_restore);
  - TF_ASSIGN_OR_RETURN(auto module, importer.ConvertSignatures());
  -
  - SortSavedModelModule(*module);
  - MarkSavedModelFunctionVisibility(*module);
  -
  - return module;
  + return importer.ConvertSignatures();
  }
  private:
  @@ -3789,6 +3783,8 @@ SavedModelSignatureDefImporterLite::ParseInputArrays(
  StatusOr<mlir::OwningModuleRef>
  SavedModelSignatureDefImporterLite::ConvertSignatures() {
  + LoadImporterDialects(*module_->getContext());
  +
  const auto& signatures = input_.meta_graph_def().signature_def();
  PopulateTfVersions(module_.get(),
  input_.meta_graph_def().graph_def().versions());
  