Revert D32786909: [C10D] [Easy] Use pinned memory for HtoD copies in Reducer:: sync_bucket_indices
Test Plan: revert-hammer
Differential Revision:
D32786909 (https://github.com/pytorch/pytorch/commit/dbc8d9c947dd9cf9527f92af4b1278c8bac245f7)
Original commit changeset: a53f96f57e67
fbshipit-source-id: 19599c3a489804bfdbb3062f4256dceb680c143b
