[oneDPL][ranges][merge] + redo __get_value() change; => __get_value(size_t idx = 0)

Author: MikeDvorskiy

include/oneapi/dpl/pstl/hetero/dpcpp/parallel_backend_sycl_utils.h

@@ -667,17 +667,10 @@ struct __result_and_scratch_storage : __result_and_scratch_storage_base
         return __get_value();
     }
 
-  private:
-    bool
-    is_USM() const
-    {
-        return __supports_USM_device;
-    }
-
     // Note: this member function assumes the result is *ready*, since the __future has already
     // waited on the relevant event.
     _T
-    __get_value() const
+    __get_value(size_t idx = 0) const
     {
         assert(__result_n == 1);
         if (__use_USM_host && __supports_USM_device)
@@ -696,6 +689,13 @@ struct __result_and_scratch_storage : __result_and_scratch_storage_base
         }
     }
 
+  private:
+    bool
+    is_USM() const
+    {
+        return __supports_USM_device;
+    }
+
     template <typename _Type>
     std::size_t
     __fill_data(std::pair<_Type, _Type>&& __p, std::size_t* __p_buf) const