[SYCL][Graph] Fix bug with debug print graph feature (#11976)

Some nodes may not have any associated CommandGroup, e.g. empty nodes. 
This PR therefore changes the way node IDs are assigned in the printed
graph.
Node IDs are now based on the Node address instead of the CommandGroup
address.
The test has been update to generate a node without a CommandGroup.

Author: mfrancepillois

sycl/source/detail/graph_impl.hpp

@@ -246,8 +246,8 @@ class node_impl {
     printDotCG(Stream, Verbose);
     for (const auto &Dep : MPredecessors) {
       auto NodeDep = Dep.lock();
-      Stream << "  \"" << NodeDep->MCommandGroup.get() << "\" -> \""
-             << MCommandGroup.get() << "\"" << std::endl;
+      Stream << "  \"" << NodeDep.get() << "\" -> \"" << this << "\""
+             << std::endl;
     }
 
     for (std::weak_ptr<node_impl> Succ : MSuccessors) {
@@ -261,14 +261,12 @@ class node_impl {
   /// @param Verbose If true, print additional information about the nodes such
   /// as kernel args or memory access where applicable.
   void printDotCG(std::ostream &Stream, bool Verbose) {
-    sycl::detail::CG::CGTYPE CGType = MCommandGroup->getType();
+    Stream << "\"" << this << "\" [style=bold, label=\"";
 
-    Stream << "\"" << MCommandGroup.get() << "\" [style=bold, label=\"";
-
-    Stream << "ID = " << MCommandGroup.get() << "\\n";
+    Stream << "ID = " << this << "\\n";
     Stream << "TYPE = ";
 
-    switch (CGType) {
+    switch (MCGType) {
     case sycl::detail::CG::CGTYPE::None:
       Stream << "None \\n";
       break;

sycl/test-e2e/Graph/Inputs/debug_print_graph.cpp

@@ -50,7 +50,7 @@ int main() {
       CGH.copy(AccA, DataB2D.data());
     });
 
-    add_node(Graph, Queue, [&](handler &CGH) { /* empty node */ });
+    add_empty_node(Graph, Queue);
 
     Graph.print_graph("graph.dot");