Move distributed_init into DistributedFairseqModel (#687)

Summary:
This should make rendezvous happen as lazily as possible.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/687

Differential Revision: D15151145

Pulled By: myleott

fbshipit-source-id: d70816a85414c5d509a6b12e2b339b4736db2c88