Use cuda memset async (#21216)

### Description
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### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->

orttraining/orttraining/training_ops/cuda/tensor/pad_and_unflatten.cc

@@ -35,7 +35,8 @@ struct PadAndUnflattenFunctor {
     typedef typename ToCudaType<T>::MappedType CudaT;
     const CudaT* input_data = reinterpret_cast<const CudaT*>(input_tensor.Data<T>());
 
-    CUDA_CALL_THROW(cudaMemset(output_tensor.MutableDataRaw(), 0, output_tensor.Shape().Size() * sizeof(CudaT)));
+    CUDA_CALL_THROW(cudaMemsetAsync(output_tensor.MutableDataRaw(), 0, output_tensor.Shape().Size() * sizeof(CudaT),
+                                    stream));
     PadAndUnflattenImpl<CudaT>(stream, input_element_count, output_element_stride_fdm, index_value_upper_bound,
                                input_data, indices_tensor.Data<int64_t>(),
                                reinterpret_cast<CudaT*>(output_tensor.MutableData<T>()));
@@ -48,6 +49,7 @@ Status PadAndUnflatten::ComputeInternal(OpKernelContext* context) const {
   const Tensor* input_tensor = context->Input<Tensor>(0);
   const Tensor* indices_tensor = context->Input<Tensor>(1);
   const Tensor* unflatten_dims_tensor = context->Input<Tensor>(2);  // Parse the 1-D shape tensor.
+
   ORT_ENFORCE(unflatten_dims_tensor->Shape().NumDimensions() == 1,
               "unflatten_dims_tensor tensor must be 1-D.", unflatten_dims_tensor->Shape().NumDimensions());
   ORT_ENFORCE(unflatten_dims_tensor->Shape().Size() == 2,