Add the check of lod in sequence_softmax kernel. (PaddlePaddle#18996)

* Add the check of lod in sequence_softmax kernel.
test=develop

* Refine the comments.
test=develop

paddle/fluid/operators/sequence_ops/sequence_softmax_op.h

@@ -97,6 +97,9 @@ class SequenceSoftmaxKernel : public framework::OpKernel<T> {
     auto dims = x->dims();
 
     const size_t level = lod.size() - 1;
+    PADDLE_ENFORCE_GT(
+        lod.size(), 0U,
+        "The LoD level of Input X should be larger than 0 (lod.size() > 0).");
     PADDLE_ENFORCE_EQ(dims[0], static_cast<int64_t>(lod[level].back()),
                       "The first dimension of Input(X) should be equal to the "
                       "sum of all sequences' lengths.");