fix softmax with cross entropy out of bound; test=develop (#25549) (#25567)

* fix softmax with cross entropy out of bound; test=develop (#25549)

* fix index error; test=develop

paddle/fluid/operators/softmax_with_cross_entropy_op.cu

@@ -28,9 +28,11 @@ __global__ void CrossEntropyGrad(T* logit_grad, const int64_t* labels,
        i += blockDim.x * gridDim.x) {
     int idx_n = i / remain;
     int idx_remain = i % remain;
-    int idx = idx_n * d + labels[i] * remain + idx_remain;
-    logit_grad[idx] -=
-        ignore_index == labels[i] ? static_cast<T>(0.) : static_cast<T>(1.);
+    int tmp = labels[i];
+    if (ignore_index != tmp) {
+      int idx = idx_n * d + tmp * remain + idx_remain;
+      logit_grad[idx] -= static_cast<T>(1.);
+    }
   }
 }