polish cross_entropy ENFORCE (#22056)

paddle/fluid/operators/math/cross_entropy.cc

@@ -53,7 +53,21 @@ class CrossEntropyFunctor<platform::CPUDeviceContext, T> {
       for (int i = 0; i < batch_size; ++i) {
         for (int j = 0; j < num_remain; j++) {
           int lbl = label_data[i * num_remain + j];
-          PADDLE_ENFORCE((lbl >= 0 && lbl < axis_dim) || lbl == ignore_index);
+          if (lbl != ignore_index) {
+            PADDLE_ENFORCE_GE(lbl, 0,
+                              platform::errors::OutOfRange(
+                                  "label value should >= 0 when label "
+                                  "value(%f) not equal to ignore_index(%f)",
+                                  lbl, ignore_index));
+            PADDLE_ENFORCE_LT(
+                lbl, axis_dim,
+                platform::errors::OutOfRange(
+                    "label value should less than the shape of axis dimension "
+                    "when label value(%f) not equal to ignore_index(%f), But "
+                    "received label value as %ld and shape of axis dimension "
+                    "is %d",
+                    lbl, ignore_index, lbl, axis_dim));
+          }
           int index = i * num_classes + lbl * num_remain + j;
           int loss_idx = i * num_remain + j;
           loss_data[loss_idx] =