make dictionary class as input for fairseq preprocess functions (#482)

Summary:
Pull Request resolved: https://github.com/pytorch/fairseq/pull/482

With this change, we can use different dictionary classes when calling build_dictionary and build_and_save_dictionary

Reviewed By: liaimi

Differential Revision: D13855100

fbshipit-source-id: 62e6db310b5f078e05c547d2671252233be7b7f0