Patch up warnings from graph_impl.hpp

Author: ianayl

sycl/source/detail/graph_impl.hpp

@@ -242,6 +242,10 @@ class node_impl : public std::enable_shared_from_this<node_impl> {
       return createCGCopy<sycl::detail::CGFillUSM>();
     case sycl::detail::CGType::PrefetchUSM:
       return createCGCopy<sycl::detail::CGPrefetchUSM>();
+    case sycl::detail::CGType::PrefetchUSMExpD2H:
+      throw sycl::exception(
+          sycl::make_error_code(errc::feature_not_supported),
+          "Prefetch from device to host is currently not supported by SYCL Graph extension.");
     case sycl::detail::CGType::AdviseUSM:
       return createCGCopy<sycl::detail::CGAdviseUSM>();
     case sycl::detail::CGType::Copy2DUSM: