Fix GPU kernel of lookup_table_op enhancement

Author: guoshengCS

paddle/operators/lookup_table_op.cu

@@ -37,7 +37,7 @@ __global__ void LookupTable(T* output, const T* table, const int64_t* ids,
     const T* tab = table + id * D;
     for (int i = idx; i < D; i += BlockDimX) {
       if (PaddingFlag) {
-        if (idx == padding_idx)
+        if (id == padding_idx)
           out[i] = static_cast<T>(0);
         else
           out[i] = tab[i];