message: Forward gradients: try to placate the Windows compiler
diff: @@ -1001,7 +1001,8 @@ Status ForwardAccumulator<Gradient, BackwardFunction, TapeTensor>::Accumulate(
  template <typename Gradient, typename BackwardFunction, typename TapeTensor>
  void ForwardAccumulator<Gradient, BackwardFunction, TapeTensor>::Watch(
  int64 tensor_id, Gradient* tangent) {
  - auto existing = accumulated_gradients_.find(tensor_id);
  + typename std::unordered_map<int64, Gradient*>::iterator existing =
  + accumulated_gradients_.find(tensor_id);
  vspace_.MarkAsResult(tangent);
  if (existing == accumulated_gradients_.end()) {
  accumulated_gradients_.emplace(tensor_id, tangent);
  