Back out "[pytorch][PR] Removes dunder div"
Summary: NVIDIA's Apex is updating to no longer rely on this behavior, but we're reverting this Python2->Python3 update to unblock internal apex users.
Test Plan: Sandcaslte + OSS CI.
Reviewed By: ngimel
Differential Revision: D22146782
fbshipit-source-id: f9483d2cbf9dc3a469ad48a6c863edea3ae51070
