message: add missing `ActivationMode::kNone` to `ActivationModeString`
diff: @@ -94,6 +94,8 @@ std::string QuantizedActivationModeString(QuantizedActivationMode mode) {
  std::string ActivationModeString(ActivationMode mode) {
  switch (mode) {
  + case ActivationMode::kNone:
  + return "none";
  case ActivationMode::kSigmoid:
  return "sigmoid";
  case ActivationMode::kRelu:
  