update checkpoint mkdir behavior (issue #1986) (#2011)
update checkpoint mkdir behavior (issue #1986) (#2011)

Summary:
Create checkpoint directory in ```save_checkpoint()```instead of ```load_checkpoint()```.

https://github.com/pytorch/fairseq/issues/1986
Pull Request resolved: https://github.com/pytorch/fairseq/pull/2011

Reviewed By: ngoyal2707

Differential Revision: D21017208

Pulled By: myleott

fbshipit-source-id: 4f76afc1751f987b8ab2c170ca306d07bd5ffe83

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq