MKLDNN conv + elementwise_add fusion: comment explaining CorrectGraphEdges added

paddle/fluid/framework/ir/conv_elementwise_add_mkldnn_fuse_pass.cc

@@ -21,6 +21,10 @@ namespace paddle {
 namespace framework {
 namespace ir {
 namespace {
+
+// The function keeps the graph consistent by replacing
+// a node 'from' in the set of inputs nodes
+// of the visited node by a node 'to'.
 void CorrectGraphEdges(Graph* graph, Node* from, Node* to) {
   for (auto& node : GraphTraits::DFS(*graph)) {
     auto from_in_inputs =