Fix save_dir creation while training on multiple nodes (#626)

Summary:
Pull Request resolved: https://github.com/pytorch/fairseq/pull/626

While training a model on multiple GPUs, the current fairseq train workflow fails while creating the directory from which to load a checkpoint. This seems to be happening because multiple nodes attempt to create the same directory thus causing some weird interaction with os.makedirs option "exist_ok=True". Fixing this by making sure only rank 0 creates this directory.

Reviewed By: myleott

Differential Revision: D14841304

fbshipit-source-id: c9b73ba804de97e2cb19a616189fefce476d8c74