Fix "non-negative integer" error messages (#42734)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/42662
Use "positive integer" error message for consistency with: https://github.com/pytorch/pytorch/blob/17f76f9a7896eccdfdba5fd22fd3a24002b0d917/torch/optim/lr_scheduler.py#L958-L959
https://github.com/pytorch/pytorch/blob/ad7133d3c11a35a7aedf9786ccf8d7a52939b753/torch/utils/data/sampler.py#L102-L104
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42734
Reviewed By: zdevito
Differential Revision: D23039575
Pulled By: smessmer
fbshipit-source-id: 1be1e0caa868891540ecdbe6f471a6cd51c40ede
