the error of backpropagation

[xuhongxin]

i was study the Section 2.5(Automatic Differentiation), but when i run the follow demo code , it report the error;what is the reason of the error?

the demo code
try (GradientCollector gc = Engine.getInstance().newGradientCollector()) { NDArray y = x.sum(); gc.backward(y); } x.getGradient() // Overwritten by the newly calculated gradient

the error
`
ai.djl.engine.EngineException: MXNet engine call failed: MXNetError: Check failed: !AGInfo: :IsNone(*i): Cannot differentiate node because it is not in a computational graph. You need to set is_recording to true or use autograd.record() to save computational graphs for backward. If you want to differentiate the same graph twice, you need to pass retain_graph=True to backward.
Stack trace:
File "/Users/runner/work/djl/djl/src/imperative/imperative.cc", line 295

at ai.djl.mxnet.jna.JnaUtils.checkCall(JnaUtils.java:1942)
at ai.djl.mxnet.jna.JnaUtils.autogradBackward(JnaUtils.java:665)
at ai.djl.mxnet.engine.MxGradientCollector.backward(MxGradientCollector.java:117)
at ai.djl.mxnet.engine.MxGradientCollector.backward(MxGradientCollector.java:106)
at ai.djl.examples.selfdemo.TableSawTest.test05_v1(TableSawTest.java:240)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:566)
at org.testng.internal.invokers.MethodInvocationHelper.invokeMethod(MethodInvocationHelper.java:139)
at org.testng.internal.invokers.TestInvoker.invokeMethod(TestInvoker.java:664)
at org.testng.internal.invokers.TestInvoker.invokeTestMethod(TestInvoker.java:227)
at org.testng.internal.invokers.MethodRunner.runInSequence(MethodRunner.java:50)
at org.testng.internal.invokers.TestInvoker$MethodInvocationAgent.invoke(TestInvoker.java:957)
at org.testng.internal.invokers.TestInvoker.invokeTestMethods(TestInvoker.java:200)
at org.testng.internal.invokers.TestMethodWorker.invokeTestMethods(TestMethodWorker.java:148)
at org.testng.internal.invokers.TestMethodWorker.run(TestMethodWorker.java:128)
at java.base/java.util.ArrayList.forEach(ArrayList.java:1541)
at org.testng.TestRunner.privateRun(TestRunner.java:848)
at org.testng.TestRunner.run(TestRunner.java:621)
at org.testng.SuiteRunner.runTest(SuiteRunner.java:443)
at org.testng.SuiteRunner.runSequentially(SuiteRunner.java:437)
at org.testng.SuiteRunner.privateRun(SuiteRunner.java:397)
at org.testng.SuiteRunner.run(SuiteRunner.java:336)
at org.testng.SuiteRunnerWorker.runSuite(SuiteRunnerWorker.java:52)
at org.testng.SuiteRunnerWorker.run(SuiteRunnerWorker.java:95)
at org.testng.TestNG.runSuitesSequentially(TestNG.java:1280)
at org.testng.TestNG.runSuitesLocally(TestNG.java:1200)
at org.testng.TestNG.runSuites(TestNG.java:1114)
at org.testng.TestNG.run(TestNG.java:1082)
at com.intellij.rt.testng.IDEARemoteTestNG.run(IDEARemoteTestNG.java:66)
at com.intellij.rt.testng.RemoteTestNGStarter.main(RemoteTestNGStarter.java:109)

`