Pickler: convert std::stringstream cases. (#29351)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29351
When torch::save()ing a smallish tensor, we spend ~5% of the time
still in std::stringstream constructors.
This removes the last couple of cases. Benchmark shows ~5% improvement:
TorchSaveSmallTensor Pre: 13.12us
TorchSaveSmallTensor Post: 12.48us
ghstack-source-id: 93517928
Test Plan:
buck build mode/opt experimental/jeremyl/c2:
buck-out/opt/gen/experimental/jeremyl/c2/SerializationBench --bm_regex=TorchSaveSmallTensor
Differential Revision: D18365066
fbshipit-source-id: a3284bec004751cedae1cdadf27f969422faff8e
