[Op] Add C++ gradient op for Atan2.

Cherry-pick from TensorFlow 5dee95f

Author: liutongxuan

tensorflow/cc/gradients/math_grad.cc

@@ -249,6 +249,19 @@ Status AtanhGrad(const Scope& scope, const Operation& op,
 }
 REGISTER_GRADIENT_OP("Atanh", AtanhGrad);
 
+Status Atan2Grad(const Scope& scope, const Operation& op,
+                 const std::vector<Output>& grad_inputs,
+                 std::vector<Output>* grad_outputs) {
+  auto y = op.input(0);
+  auto x = op.input(1);
+  Output grad_inv = Div(scope, grad_inputs[0],
+                        Add(scope, Square(scope, x), Square(scope, y)));
+  grad_outputs->push_back(Mul(scope, x, grad_inv));
+  grad_outputs->push_back(Mul(scope, Neg(scope, y), grad_inv));
+  return scope.status();
+}
+REGISTER_GRADIENT_OP("Atan2", Atan2Grad);
+
 Status SigmoidGrad(const Scope& scope, const Operation& op,
                    const std::vector<Output>& grad_inputs,
                    std::vector<Output>* grad_outputs) {

tensorflow/cc/gradients/math_grad_test.cc

@@ -31,6 +31,7 @@ using ops::Abs;
 using ops::Add;
 using ops::AddN;
 using ops::AddV2;
+using ops::Atan2;
 using ops::BatchMatMul;
 using ops::Cast;
 using ops::Const;
@@ -1015,5 +1016,13 @@ TEST_F(NaryGradTest, SelectV2_Broadcast) {
   RunTest({x, y}, {x_shape, y_shape}, {z}, {x_shape});
 }
 
+TEST_F(NaryGradTest, Atan2Grad) {
+  TensorShape shape({3, 2, 5});
+  auto x1 = Placeholder(scope_, DT_FLOAT, Placeholder::Shape(shape));
+  auto x2 = Placeholder(scope_, DT_FLOAT, Placeholder::Shape(shape));
+  auto y = Atan2(scope_, x1, x2);
+  RunTest({x1, x2}, {shape, shape}, {y}, {shape});
+}
+
 }  // namespace
 }  // namespace tensorflow