message: Ensure space of tensors in out_vector are reserved.
diff: @@ -235,6 +235,7 @@ class BigQueryReaderArrowDatasetIterator
  const std::vector<DataType> &output_types)
  TF_EXCLUSIVE_LOCKS_REQUIRED(this->mu_) override {
  out_tensors->clear();
  + out_tensors->reserve(columns.size());
  if (this->current_row_index_ == 0) {
  this->column_indices_.clear();
  @@ -413,6 +414,7 @@ class BigQueryReaderAvroDatasetIterator
  }
  out_tensors->clear();
  + out_tensors->reserve(columns.size());
  for (size_t i = 0; i < columns.size(); i++) {
  Tensor tensor(ctx->allocator({}), output_types[i], {});
  out_tensors->emplace_back(std::move(tensor));
  