message: Give names to TensorId (std::pair) values.
diff: @@ -41,6 +41,9 @@ struct TensorId : public std::pair<StringPiece, int> {
  TensorId() : Base() {}
  TensorId(const SafeTensorId& id);
  + const StringPiece node() const { return first; }
  + int index() const { return second; }
  +
  string ToString() const {
  if (second == Graph::kControlSlot) return strings::StrCat("^", first);
  return strings::StrCat(first, ":", second);
  @@ -68,6 +71,9 @@ struct SafeTensorId : public std::pair<string, int> {
  SafeTensorId(const string& str, int idx) : Base(str, idx) {}
  SafeTensorId(const TensorId& id);
  + const string& node() const { return first; }
  + int index() const { return second; }
  +
  string ToString() const {
  if (second == Graph::kControlSlot) return strings::StrCat("^", first);
  return strings::StrCat(first, ":", second);
  