message: [ParseExample] Use `const tstring&` in FastParseSerializedExample().
  This should avoid a temporary string copy when USE_TSTRING is enabled.
diff: @@ -546,7 +546,7 @@ void LogSparseFeatureDataLoss(StringPiece feature_name) {
  }
  Status FastParseSerializedExample(
  - const string& serialized_example, const string& example_name,
  + const tstring& serialized_example, const tstring& example_name,
  const size_t example_index, const Config& config,
  const PresizedCuckooMap<std::pair<size_t, Type>>& config_index,
  SeededHasher hasher, std::vector<Tensor>* output_dense,
  