Beam search

[kylebgorman]

Beam search is generally not supported by our models, though the flag exists. It appears to be supported in lstm; it is unclear to me whether it's supported by feature_invariant_transformer, transformer and transducer (if any of these silently ignore the flag, which is what I think happens, this is a serious but easy-to-fix bug), and it's explicitly not implemented in pointer_generator_lstm.