Replace std::shared_ptr::unique() with use_count() == 1

std::shared_ptr::unique() has been removed from c++20
PiperOrigin-RevId: 631443441

tensorflow/core/graph/graph.cc

@@ -270,7 +270,8 @@ gtl::iterator_range<NeighborIter> Node::in_nodes() const {
 void Node::MaybeCopyOnWrite() {
   // TODO(mdan): As nodes become more dynamic, this may not be worth the cost.
   // NodeProperties may be shared between Nodes. Make a copy if so.
-  if (!props_.unique()) {
+  // TODO(b/338453606): use_count() == 1 has a race condition.
+  if (!(props_.use_count() == 1)) {
     props_ = std::make_shared<NodeProperties>(*props_);
   }
 }