add get_local_solver in schwarz

Author: yhmtsai

include/ginkgo/core/distributed/preconditioner/schwarz.hpp

@@ -113,6 +113,11 @@ class Schwarz
             config::make_type_descriptor<ValueType, LocalIndexType,
                                          GlobalIndexType>());
 
+    std::shared_ptr<const LinOp> get_local_solver() const
+    {
+        return local_solver_;
+    }
+
 protected:
     /**
      * Creates an empty Schwarz preconditioner.