add an enforce

paddle/operators/sequence_concat_op.cc

@@ -65,7 +65,8 @@ class SequenceConcatOpMaker : public framework::OpProtoAndCheckerMaker {
                  "If the level is 0, the inputs will be joined at the nested "
                  "sequence level."
                  "If the level is 1, the inputs will be joined at the "
-                 "sequence level.")
+                 "sequence level."
+                 "The level should be less than the levels of input")
         .SetDefault(0);
     AddComment(R"DOC(
     The sequence_concat operator concatenates multiple LoDTensors. 

paddle/operators/sequence_concat_op.h

@@ -77,6 +77,9 @@ class SequenceConcatOpKernel : public framework::OpKernel<T> {
                           "LoDTensors should be the same.");
       }
     }
+    PADDLE_ENFORCE_GT(ins[0]->NumLevels(), level,
+                      "The levels of all the input LoDTensors "
+                      "should be greater than the specify level");
 
     out->mutable_data<T>(ctx.GetPlace());
     auto out_lod = concatLoD<LoDTensor>(ins, axis, level);