message: Remove unnecessary string copy.
diff: @@ -52,7 +52,8 @@ class SummaryScalarOp : public OpKernel {
  Summary s;
  for (int i = 0; i < Ttags.size(); i++) {
  Summary::Value* v = s.add_value();
  - v->set_tag(string(Ttags(i))); // NOLINT
  + const tstring& Ttags_i = Ttags(i);
  + v->set_tag(Ttags_i.data(), Ttags_i.size());
  v->set_simple_value(float(Tvalues(i)));
  }
  @@ -102,7 +103,8 @@ class SummaryHistoOp : public OpKernel {
  Summary s;
  Summary::Value* v = s.add_value();
  - v->set_tag(string(tags.scalar<tstring>()())); // NOLINT
  + const tstring& tags0 = tags.scalar<tstring>()();
  + v->set_tag(tags0.data(), tags0.size());
  histo.EncodeToProto(v->mutable_histo(), false /* Drop zero buckets */);
  Tensor* summary_tensor = nullptr;
  