Fix missing import sys in pin_memory.py (#19419)
Summary:
kostmo pointed this out in #15331. Thanks :)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19419
Differential Revision: D15002846
Pulled By: soumith
fbshipit-source-id: c600fab3f7a7a5147994b9363910af4565c7ee65
