Fix warnings in lookup_op

paddle/operators/lookup_table_op.h

@@ -30,12 +30,12 @@ class LookupTableKernel : public framework::OpKernel {
     auto ids_t = context.Input<Tensor>("Ids");      // int tensor
     auto output_t = context.Output<Tensor>("Out");  // float tensor
 
-    size_t N = table_t->dims()[0];
-    size_t D = table_t->dims()[1];
+    int N = table_t->dims()[0];
+    int D = table_t->dims()[1];
     auto ids = ids_t->data<int32_t>();
     auto table = table_t->data<T>();
     auto output = output_t->mutable_data<T>(context.GetPlace());
-    for (size_t i = 0; i < product(ids_t->dims()); ++i) {
+    for (ssize_t i = 0; i < product(ids_t->dims()); ++i) {
       PADDLE_ENFORCE_LT(ids[i], N);
       PADDLE_ENFORCE_GE(ids[i], 0);
       memcpy(output + i * D, table + ids[i] * D, D * sizeof(T));
@@ -51,8 +51,8 @@ class LookupTableGradKernel : public framework::OpKernel {
     auto d_output_t = context.Input<Tensor>(framework::GradVarName("Out"));
     auto d_table_t = context.Output<Tensor>(framework::GradVarName("W"));
 
-    size_t N = d_table_t->dims()[0];
-    size_t D = d_table_t->dims()[1];
+    int N = d_table_t->dims()[0];
+    int D = d_table_t->dims()[1];
     auto ids = ids_t->data<int32_t>();
     const T* d_output = d_output_t->data<T>();
     T* d_table = d_table_t->mutable_data<T>(context.GetPlace());
@@ -61,10 +61,10 @@ class LookupTableGradKernel : public framework::OpKernel {
     t.device(context.GetEigenDevice<platform::CPUPlace>()) =
         t.constant(static_cast<T>(0));
 
-    for (size_t i = 0; i < product(ids_t->dims()); ++i) {
+    for (ssize_t i = 0; i < product(ids_t->dims()); ++i) {
       PADDLE_ENFORCE_LT(ids[i], N);
       PADDLE_ENFORCE_GE(ids[i], 0);
-      for (size_t j = 0; j < D; ++j) {
+      for (int j = 0; j < D; ++j) {
         d_table[ids[i] * D + j] += d_output[i * D + j];
       }
     }