message: Fix compiler errors with some compilers
  Test Plan: all tests pass.
  Reviewers: jakeh, davidn
diff: @@ -129,8 +129,7 @@ poplar::Tensor DoCachedBatchNormInference(
  using namespace poputil::graphfn;
  auto f = TensorFunction(
  - graph,
  - {input(operand, "operand"), input(scale, "scale"),
  + graph, {input(operand, "operand"), input(scale, "scale"),
  input(offset, "offset"), input(mean, "mean"),
  input(variance, "variance")},
  [&](std::vector<poplar::Tensor>& args, poplar::program::Sequence& seq) {
  @@ -159,13 +158,12 @@ DoCachedBatchNormTraining(poplar::Graph& graph, CompilerResources& res,
  !res.disable_graph_convolution_caching) {
  auto& f = it->second;
  f(args, prog);
  - return {args[3], args[4], args[5]};
  + return std::make_tuple(args[3], args[4], args[5]);
  }
  using namespace poputil::graphfn;
  auto f = VoidFunction(
  - graph,
  - {input(operand, "operand"), input(scale, "scale"),
  + graph, {input(operand, "operand"), input(scale, "scale"),
  input(offset, "offset"), created("output"), created("mean"),
  created("variance")},
  [&](std::vector<poplar::Tensor>& args, poplar::program::Sequence& seq) {
  @@ -181,7 +179,7 @@ DoCachedBatchNormTraining(poplar::Graph& graph, CompilerResources& res,
  });
  res.bn_tr_graph_cache.emplace(key, f);
  f(args, prog);
  - return {args[3], args[4], args[5]};
  + return std::make_tuple(args[3], args[4], args[5]);
  }
  std::tuple<poplar::Tensor, poplar::Tensor, poplar::Tensor>
  @@ -203,7 +201,7 @@ DoCachedBatchNormGrad(poplar::Graph& graph, CompilerResources& res,
  !res.disable_graph_convolution_caching) {
  auto& f = it->second;
  f(args, prog);
  - return {args[5], args[6], args[7]};
  + return std::make_tuple(args[5], args[6], args[7]);
  }
  using namespace poputil::graphfn;
  auto f = VoidFunction(
  @@ -230,7 +228,7 @@ DoCachedBatchNormGrad(poplar::Graph& graph, CompilerResources& res,
  });
  res.bn_grad_graph_cache.emplace(key, f);
  f(args, prog);
  - return {args[5], args[6], args[7]};
  + return std::make_tuple(args[5], args[6], args[7]);
  }
  } // namespace batch_norm_graph_caching
  