fix warning

paddle/fluid/operators/cross_entropy_op.h

@@ -86,8 +86,8 @@ class XeGradFunctor {
     auto x_is_true_offset = sample_id * num_classes_ + label_[sample_id];
     for (size_t x_offset = sample_id * num_classes_;
          x_offset < (sample_id + 1) * num_classes_; ++x_offset) {
-      dx_[x_offset] =
-          (x_offset != x_is_true_offset || label_[sample_id] == ignore_index_)
+      dx_[x_offset] = (x_offset != x_is_true_offset ||
+                       label_[sample_id] == static_cast<int64_t>(ignore_index_))
                           ? static_cast<T>(0)
                           : -dy_[sample_id] / x_[x_offset];
     }