Plain LRN op throws an exception when is_test is set in backward pass

paddle/fluid/operators/lrn_op.cc

@@ -214,7 +214,10 @@ class LRNOpMaker : public framework::OpProtoAndCheckerMaker {
         "Defaults to \"NHWC\". Specify the data format of the output data, "
         "the input will be transformed automatically. ")
         .SetDefault("AnyLayout");
-    AddAttr<bool>("is_test", "").SetDefault(false);
+    AddAttr<bool>("is_test",
+                  "Turns on memory optimization that optimizes away "
+                  "unnecessary memory allocations. Used by MKLDNN.")
+        .SetDefault(false);
 
     AddComment(R"DOC(
 Local Response Normalization Operator.

paddle/fluid/operators/lrn_op.h

@@ -121,6 +121,10 @@ class LRNGradKernel : public framework::OpKernel<T> {
     T alpha = ctx.Attr<T>("alpha");
     T beta = ctx.Attr<T>("beta");
 
+    PADDLE_ENFORCE(
+        !ctx.Attr<bool>("is_test"),
+        "is_test attribute should be set to False in training phase.");
+
     LRNGradFunctor<DeviceContext, T> f;
     f(ctx, x, out, mid, x_g, out_g, N, C, H, W, n, alpha, beta);
   }