message: Fix unused variable warning when CUDNN_VERSION < 7603
diff: @@ -3924,14 +3924,14 @@ port::Status CudnnSupport::DoPrepareForCtcLoss(
  absl::Span<const int> input_lengths_data,
  ScratchAllocator* scratch_allocator, DeviceMemory<uint8>* scratch_memory) {
  auto cudnn = cudnn_->GetHandle(parent_, stream);
  + // Query the workspace size.
  + size_t workspace_size_in_bytes = 0;
  +#if CUDNN_VERSION >= 7603
  CudnnCtcLossDescriptor cudnn_ctc_loss_desc(ToCudnnDataType(element_type));
  const CudnnRnnStateTensorDescriptor& cudnn_probs_desc =
  static_cast<const CudnnRnnStateTensorDescriptor&>(probs_desc);
  const CudnnRnnStateTensorDescriptor& cudnn_grads_desc =
  static_cast<const CudnnRnnStateTensorDescriptor&>(grads_desc);
  - // Query the workspace size.
  - size_t workspace_size_in_bytes = 0;
  -#if CUDNN_VERSION >= 7603
  RETURN_IF_CUDNN_ERROR(cudnnGetCTCLossWorkspaceSize(
  /*handle=*/cudnn.handle(), /*probsDesc=*/cudnn_probs_desc.handle(),
  /*gradientsDesc=*/cudnn_grads_desc.handle(),
  