Fix a segfault while printing shapes for multi-output ops in VisualizeTree (#677)

* fix a segfault while printing shapes for multi-output ops

src/ngraph/pass/visualize_tree.cpp

@@ -103,7 +103,10 @@ std::string pass::VisualizeTree::get_attributes(shared_ptr<Node> node)
 
     if (std::getenv("NGRAPH_VISUALIZE_TREE_OUTPUT_SHAPES") != nullptr)
     {
-        ss << " " << vector_to_string(node->get_shape());
+        // The shapes of the Outputs of a multi-output op
+        // will be printed for its corresponding `GetOutputElement`s
+        ss << " " << (node->get_outputs().size() != 1 ? std::string("[skipped]")
+                                                      : vector_to_string(node->get_shape()));
     }
 
     ss << " \"]\n";