Rewrite model frontend to be json-driven.

Change-Id: Iac786eff96183938d2fd11cde9313c6e8e1270a5
diff --git a/examples/test_add_1x4x4x4_f32/model.pb b/examples/test_add_1x4x4x4_f32/model.pb
index c2570bf..930928d 100644
--- a/examples/test_add_1x4x4x4_f32/model.pb
+++ b/examples/test_add_1x4x4x4_f32/model.pb
@@ -70,7 +70,7 @@
 }
 node {
   name: "result"
-  op: "Add"
+  op: "AddV2"
   input: "placeholder_0"
   input: "placeholder_1"
   attr {
@@ -92,5 +92,5 @@
   }
 }
 versions {
-  producer: 714
+  producer: 772
 }