pin_memory thread now uses 1 thread only (#25111)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/25010
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25111
Differential Revision: D16992718
Pulled By: soumith
fbshipit-source-id: fe23721d4cc293fa245c84c656241730335077dd
