fix to skip node not in graph. (apache#5238)

fix to skip node not in graph because some network cannot be hybridized with some var unused.

nnvm/src/core/graph.cc

@@ -98,7 +98,7 @@ IndexedGraph::IndexedGraph(const Graph &g) {
       // input entries
       for (const auto& e : n->inputs) {
         auto it = node2index_.find(e.node.get());
-        CHECK(it != node2index_.end() && it->first == e.node.get());
+        if (it == node2index_.end() || it->first != e.node.get()) continue;
         input_entries_.emplace_back(NodeEntry{it->second, e.index, e.version});
       }
       inputs_rptr.push_back(input_entries_.size());