add check to make sure we don't replace unreachable nodes (#1039)

* add assert to make sure we don't replace unreachable nodes

* fix unittest failures

* sparsity fix

src/ngraph/graph_util.cpp

@@ -112,6 +112,11 @@ void ngraph::replace_node(std::shared_ptr<Node> target, std::shared_ptr<Node> re
         throw ngraph_error("Result nodes cannot be replaced.");
     }
 
+    if (target->get_users().size() == 0)
+    {
+        throw ngraph_error("replacing an unreachable node");
+    }
+
     // Fix input/output descriptors
     assert(target->get_outputs().size() == replacement->get_outputs().size());
 

src/ngraph/pass/core_fusion.cpp

@@ -261,6 +261,13 @@ void pass::CoreFusion::construct_optimized_strided_conv()
         NGRAPH_DEBUG << "In a callback for construct_conv_skip against "
                      << m.get_match_root()->get_name();
 
+        if (m.get_match_root()->get_users().empty())
+        {
+            NGRAPH_DEBUG << m.get_match_root()
+                         << " has already been replaced by a preceding callback";
+            return false;
+        }
+
         auto pattern_map = m.get_pattern_map();
         auto m_eltwise = pattern_map[eltwise_label];
         auto strided_convs = m_eltwise->get_users();

src/ngraph/runtime/cpu/pass/cpu_concat_inputs.cpp

@@ -111,12 +111,15 @@ void ngraph::runtime::cpu::pass::ConcatInputs::concat_lstm_inputs()
             {
                 auto goe_node = std::dynamic_pointer_cast<op::GetOutputElement>(goes->get_node());
                 lstm_outputs.insert(goes->get_node());
-                // first output node of lstm
+                //first output node of lstm
                 if (goe_node->get_n() == 0)
                 {
                     NGRAPH_DEBUG << "Replacing 1st output Lstm node " << goe_node->get_name()
                                  << " with " << lstm_ht_out->get_name();
-                    ngraph::replace_node(goe_node, lstm_ht_out);
+                    if (goe_node->get_users().size() > 0)
+                    {
+                        ngraph::replace_node(goe_node, lstm_ht_out);
+                    }
                 }
                 else if (goe_node->get_n() == 1)
                 {

src/ngraph/runtime/cpu/pass/cpu_rnn_fusion.cpp

@@ -815,6 +815,10 @@ void ngraph::runtime::cpu::pass::MultiLayerRNNFusion::construct_multi_layer_rnn_
         for (auto& rnn_goes : rnn_node->get_users())
         {
             NGRAPH_DEBUG << "rnn_goes: " << rnn_goes->get_name();
+            if (rnn_goes->get_users().size() == 0)
+            {
+                continue;
+            }
             if (auto rnn_goe_node = std::dynamic_pointer_cast<op::GetOutputElement>(rnn_goes))
             {
                 if (rnn_goe_node->get_n() == 0)