Add diversity to the beam search

[leopd]

Perhaps with Diverse Beam Search, Vijayakumar, et al 2016

[leopd]

Useful diversity was added in 1260366 but it's not using the diverse beam search (DBS) algorithm, which keeps separate beams isolated from each other. DBS would probably still improve over this, because this still tends to converge to a small number of prefixes on long sentences even with a large diversity penalty (like 100).