Revert D22162469: [pytorch][PR] Migrate `var` & `std` to ATen
Test Plan: revert-hammer
Differential Revision:
D22162469 (https://github.com/pytorch/pytorch/commit/7a3c223bbb711c7a93910ce406a0126b8000b43b)
Original commit changeset: 8d901c779767
fbshipit-source-id: 9e0fa439732478349c0ac6c7baafba063edfac5d
