Add default noising argument in WordNoiser initialization (#664)

Summary:
Pull Request resolved: https://github.com/pytorch/fairseq/pull/664

Previously arguments for noising (dropout_prob for WordDropout and max_shuffle_distance for WordShuffle) are only passed in noising() so it could not be customized in NoisingDataset.

Now add default argument in initializer so the value could be specified at construction.

Reviewed By: liezl200

Differential Revision: D15071632

fbshipit-source-id: 59a9bf5a5e6d03c1e74f1b31c1927e221cb11dfa