negative label in softmax cross entropy (#36891)

paddle/fluid/operators/softmax_with_cross_entropy_op.cu

@@ -73,6 +73,9 @@ __global__ void CrossEntropyHardLabel(T* loss, const T* softmax,
 
   // thread ids compute loss[ids] using softmax[idx]
   if (ids < n * d) {
+    if (labels[ids] < 0) {  // label is negative
+      loss[ids] = static_cast<T>(0.0);
+    } else {  // label is positive of zero
       int64_t idx = idx_n * dim * d + labels[ids] * d + idx_d;
       if (IgnoreIndex == true) {
         // IgnoreIndex is true
@@ -86,6 +89,7 @@ __global__ void CrossEntropyHardLabel(T* loss, const T* softmax,
         loss[ids] = -Log(softmax[idx]);
       }
     }
+  }
 }
 
 /*