[PyTorch/Distributed]Only sync buffers when broadcast_buffers is True (#100729)
Summary: Disable buffers sync in _sync_module_states(...) when broadcast_buffers is False. This change will memory usage when a model has huge buffers and does not need broadcast buffers.
Test Plan: .
Differential Revision: D45610709
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100729
Approved by: https://github.com/mrshenli
