code: public static int cudnnGetReductionWorkspaceSize(
          cudnnHandle handle, 
          cudnnReduceTensorDescriptor reduceTensorDesc, 
          cudnnTensorDescriptor aDesc, 
          cudnnTensorDescriptor cDesc, 
          long[] sizeInBytes)
      {
          return checkResult(cudnnGetReductionWorkspaceSizeNative(handle, reduceTensorDesc, aDesc, cDesc, sizeInBytes));
      }
docs: Helper function to return the minimum size of the workspace to be passed to the reduction given the input and output
  tensors