message: improve comment describing segmentation algorithm
diff: @@ -146,8 +146,14 @@ tensorflow::Status SegmentGraph(
  node_segments.emplace_back(node);
  }
  - // Visit nodes in reverse topological order and use edge
  - // contraction to merge candidate nodes.
  + // The segmentation algorithm below visits nodes in reverse
  + // topological order and attempts to merge nodes along output
  + // edges. That means that subgraphs grow from the output-side of the
  + // network towards the inputs. In general this is not guaranteed to
  + // produce a globally optimal segmentation. In the future if we have
  + // a measure of how beneficial it is to include a given node in a
  + // TRT subgraph then we can revisit this algorithm to take advantage
  + // of that information.
  std::vector<tensorflow::Node*> order;
  tensorflow::GetPostOrder(graph, &order);
  