Revert D28677383: [DDP] Merge work and future_work in reducer
Test Plan: revert-hammer
Differential Revision:
D28677383 (https://github.com/pytorch/pytorch/commit/f8bebade4799912b97f7af3f1c8e35337d69ecf5)
Original commit changeset: 85e0620378b7
fbshipit-source-id: ef3c65b88c375aa9a6befe2ab004ec37ae7eb587
Author
Rong Rong (AI Infra)
