Use customGrad in exposed GPGPUProgram tests

src/kernels/webgl/webgl_custom_op_test.ts

@@ -55,20 +55,18 @@ describeWithFlags('custom-op webgl', WEBGL_ENVS, () => {
   }
 
   function squareAndAdd<T extends tf.Tensor>(x: T): T {
-    const webglBackend = tf.ENV.backend as tf.webgl.MathBackendWebGL;
-    const program = new SquareAndAddKernel(x.shape);
-    const backpropProgram = new SquareAndAddBackpropKernel(x.shape);
+    const fn = tf.customGrad(x => {
+      const webglBackend = tf.ENV.backend as tf.webgl.MathBackendWebGL;
+      const program = new SquareAndAddKernel(x.shape);
+      const backpropProgram = new SquareAndAddBackpropKernel(x.shape);
 
-    const forward = () => webglBackend.compileAndRun(program, [x]);
+      const value = webglBackend.compileAndRun(program, [x]);
 
-    const backward = (dy: T) => {
-      return {
-        x: () => webglBackend.compileAndRun(backpropProgram, [x]).mul(dy) as T
-      };
-    };
-
-    const res = tf.ENV.engine.runKernel(forward, {x}, backward);
-    return res as T;
+      const gradFunc = (dy: T) =>
+          webglBackend.compileAndRun(backpropProgram, [x]).mul(dy) as T;
+      return {value, gradFunc};
+    });
+    return fn(x) as T;
   }
 
   it('lets users use custom operations', () => {