call istringstream clear after str (#16820)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16820
Sometimes parsing histogram was not working correctly due to changes in D13633256
We need to call istringstream clear after str
Reviewed By: csummersea
Differential Revision: D13977509
fbshipit-source-id: ce3e8cb390641d8f0b5c9a7d6d6daadffeddbe11
