Enable custom sampling strategy in MultiCorpusSampledDataset (#639)

Summary:
Pull Request resolved: https://github.com/pytorch/fairseq/pull/639

Add argument sampling_func in the constructor to enable custom sampling over a list of dataset keys. The default strategy is to sample uniformly as it did previously.

Reviewed By: liezl200

Differential Revision: D14965774

fbshipit-source-id: f3285688a9ae3729c0ba12c22254c1144d0eea9e