url download bugfix for URLs served without Content-Length header (#16153)
Summary:
Some HTTP servers dont return Content-Length, account for that
Fixes: https://github.com/pytorch/pytorch/issues/16152
Differential Revision: D13858882
Pulled By: soumith
fbshipit-source-id: e4293e9368ed4c87548d22adec1ce0c25ea4bd8f
