Fix bug in Tape example

Author: saxenasaurabh

rfcs/20201201-cpp-gradients.md

@@ -234,7 +234,11 @@ Status ExpGradModel(AbstractContext* ctx,
   std::vector<AbstractTensorHandle*> exp_outputs(1);
   TF_RETURN_IF_ERROR(ops::Exp(ctx, inputs, absl::MakeSpan(exp_outputs), "Exp"));
   std::unique_ptr<GradientFunction> gradient_function;
-  TF_RETURN_IF_ERROR(registry.Lookup("Exp", &gradient_function));
+  ForwardOperation forward_op;
+  forward_op.op_name = “Exp”;
+  forward_op.inputs.push_back(inputs[0]);
+  forward_op.outputs.push_back(outputs[0]);
+  TF_RETURN_IF_ERROR(registry.Lookup(forward_op, &gradient_function));
   tape.RecordOperation(inputs, exp_outputs, gradient_function.release());
   TF_RETURN_IF_ERROR(tape.ComputeGradient(ctx,
                                            /*targets*/exp_outputs,