Add --checkpoint-upper-bound to average_checkpoints.py (#452)

Summary:
This is useful for averaging the last N checkpoints, ending at some "best" checkpoint.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/452

Differential Revision: D13695407

Pulled By: myleott

fbshipit-source-id: 5d9d2bff3706834f01501e9259834c77fb335817