Revert D26720919: [Gradient Compression] Remove some low-level methods of GradBucket class
Test Plan: revert-hammer
Differential Revision:
D26720919 (https://github.com/pytorch/pytorch/commit/521e1e83ea70400854e64eddb5fc97aeb73b4230)
Original commit changeset: 46fb64230087
fbshipit-source-id: e2b68892d1735b7249b4d36f3dff57160c9cbc78
