fix stack grad gpu (#32781)

paddle/fluid/operators/stack_op.cu

@@ -96,9 +96,10 @@ class StackGPUKernel : public framework::OpKernel<T> {
 };
 
 template <typename T, typename IntType>
-__global__ void UnStackCUDAKernel(const T* __restrict__ input, int pre_dim_size,
-                                  int split_dim_size, int suf_dim_size,
-                                  int num_split, T** output_ptrs) {
+__global__ void UnStackHelperCUDAKernel(const T* __restrict__ input,
+                                        int pre_dim_size, int split_dim_size,
+                                        int suf_dim_size, int num_split,
+                                        T** output_ptrs) {
   assert(blockDim.y == 1);
   assert(blockDim.z == 1);
   // In this case they are equal
@@ -114,6 +115,9 @@ __global__ void UnStackCUDAKernel(const T* __restrict__ input, int pre_dim_size,
     IntType k = offset % suf_dim_size;
 
     T* output = output_ptrs[j / each_dim_size];
+    if (output == nullptr) {
+      return;
+    }
     IntType output_ind = i * each_dim_size * suf_dim_size +
                          (j % each_dim_size) * suf_dim_size + k;
     *(output + output_ind) = input[offset];
@@ -142,6 +146,9 @@ class StackGradGPUKernel : public framework::OpKernel<T> {
     std::vector<T*> outputs(n);
     auto out_var_names = ctx.OutputNames(framework::GradVarName("X"));
     for (size_t j = 0; j < dx.size(); ++j) {
+      if (dx[j] == nullptr) {
+        outputs[j] = nullptr;
+      }
       if (out_var_names[j] != framework::kEmptyVarName &&
           dx[j]->numel() != 0UL) {
         T* ptr = dx[j]->mutable_data<T>(ctx.GetPlace());
@@ -170,13 +177,13 @@ class StackGradGPUKernel : public framework::OpKernel<T> {
     auto config = GetGpuLaunchConfig1D(dev_ctx, dy_pre * split_dim * dy_suf);
 
     if (dy->numel() < std::numeric_limits<int32_t>::max()) {
-      UnStackCUDAKernel<
+      UnStackHelperCUDAKernel<
           T, int32_t><<<config.block_per_grid.x, config.thread_per_block.x, 0,
                         dev_ctx.stream()>>>(
           dy_data, dy_pre, split_dim, dy_suf, split_dim,
           reinterpret_cast<T**>(tmp_out_data->ptr()));
     } else {
-      UnStackCUDAKernel<
+      UnStackHelperCUDAKernel<
           T, int64_t><<<config.block_per_grid.x, config.thread_per_block.x, 0,
                         dev_ctx.stream()>>>(
           dy_data, dy_pre, split_dim, dy_suf, split_dim,