message: Fix compilation error
  Test Plan: it compiles.
  Reviewers: davidn, jakeh
diff: @@ -41,8 +41,9 @@ namespace {
  BwdWeightCacheKey GetBwdWeightCacheKey(const poplar::Tensor& weights,
  const poplar::Tensor& bwd_weights,
  const uint64 device_id) {
  - return {graph_caching_util::GetPoplarTensorSignature(weights),
  - graph_caching_util::GetPoplarTensorSignature(bwd_weights), device_id};
  + return std::make_tuple(
  + graph_caching_util::GetPoplarTensorSignature(weights),
  + graph_caching_util::GetPoplarTensorSignature(bwd_weights), device_id);
  }
  void CreateCachedBwdWeights(poplar::Graph& graph, CompilerResources& res,
  