Use unsafe_get in nnvm (#2247)

dmlc-core @ 519d013a

-Subproject commit 4d49691f1a9d944c3b0aa5e63f1db3cad1f941f8
+Subproject commit 519d013a213c0c447a971f51219473ef564d2348

nnvm/include/nnvm/base.h

@@ -25,6 +25,9 @@ using dmlc::array_view;
 /*!\brief getter function of any type */
 using dmlc::get;
 
+/*!\brief "unsafe" getter function of any type */
+using dmlc::unsafe_get;
+
 }  // namespace nnvm
 
 // describe op registration point

nnvm/include/nnvm/graph.h

@@ -229,7 +229,7 @@ inline const T& Graph::GetAttr(const std::string& attr_name) const {
   auto it = attrs.find(attr_name);
   CHECK(it != attrs.end())
       << "Cannot find attribute " << attr_name << " in the graph";
-  return nnvm::get<T>(*it->second);
+  return nnvm::unsafe_get<T>(*it->second);
 }
 
 inline bool Graph::HasAttr(const std::string& attr_name) const {