'Creates BeamSearch object.
Args:
model: Seq2SeqAttentionModel.
beam_size: int.
start_token: int, id of the token to start decoding with
end_token: int, id of the token that completes an hypothesis
max_steps: int, upper limit on the size of the hypothesis'

def __init__(self, model, beam_size, start_token, end_token, max_steps):

    self._model = model
    self._beam_size = beam_size
    self._start_token = start_token
    self._end_token = end_token
    self._max_steps = max_steps
