message: Include AddN in gradient exclusion list
diff: @@ -2957,6 +2957,7 @@ bool OpGradientDoesntRequireOutputIndices(
  {"Cos", {true, {}}},
  {"Tan", {true, {}}},
  {"Add", {true, {}}},
  + {"AddN", {true, {}}},
  {"AddV2", {true, {}}},
  {"Sub", {true, {}}},
  {"Mul", {true, {}}},
  