tweak tqdm to have download speed in kB/MB/etc (#20908)
Summary:
This changes the progress bars in `_download_url_to_file` from saying things like `49773343.40it/s` to `47.5MB/s`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20908
Differential Revision: D15511223
Pulled By: soumith
fbshipit-source-id: 2422eb5fb486f9ef4bd69c556c4ed1775b8b2860
