Eval and log on a subset of directions for multimodel training (#605)

Summary:
Pull Request resolved: https://github.com/pytorch/fairseq/pull/605

Eval and log on a subset of directions for multimodel training

This reduces code duplication in PyTorch Translate's semi_supervised task and will enable clean multitask setups in the future.

Reviewed By: pipibjc, dpacgopinath

Differential Revision: D14672779

fbshipit-source-id: 1342c71781f0824cc56a38ad1c1822e34eaef337