offload state_dict to cpu
offload state_dict to cpu

Summary:
state_dict is summoned during checkpoint to GPU0. unfortunately with large models this will exceed single GPU memory limit. Moving it to cpu.
Question for Myle: should we set this option as default? when saving checkpoint the state_dict would eventually be moved to CPU any way.

Reviewed By: myleott

Differential Revision: D28203587

fbshipit-source-id: e738f48c83e35873c46bcec3471d105f2b4f4d8e

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq