COMPMID-1750: Add PriorBox operator to graph API.

Change-Id: I5d2ed5dcc342abff8124762f7bdee587cdf20032
diff --git a/arm_compute/graph/INodeVisitor.h b/arm_compute/graph/INodeVisitor.h
index a21c9b2..2df2574 100644
--- a/arm_compute/graph/INodeVisitor.h
+++ b/arm_compute/graph/INodeVisitor.h
@@ -111,6 +111,11 @@
      * @param[in] n Node to visit.
      */
     virtual void visit(PoolingLayerNode &n) = 0;
+    /** Visit PriorBoxLayerNode.
+     *
+     * @param[in] n Node to visit.
+     */
+    virtual void visit(PriorBoxLayerNode &n) = 0;
     /** Visit ReshapeLayerNode.
      *
      * @param[in] n Node to visit.
@@ -201,6 +206,10 @@
     {
         default_visit();
     }
+    virtual void visit(PriorBoxLayerNode &n) override
+    {
+        default_visit();
+    }
     virtual void visit(ReshapeLayerNode &n) override
     {
         default_visit();