Created by: guoshengCS

Fix and enhance beam_search_op and beam_searc_decode_op

Refer the source sentence as beam and the hypothesis as branch, then some points of the fix and enhancement are as following:

• beam_search_op:
  • The ended branch should not be pruned since it still might be sifted out because of low score.
  • Force the prediction of ended branch to allocate all probability mass to end token.
  • Though the ended branch cann't be pruned , the ended beam whose branches are all finished can be pruned.
• beam_search_decode_op:
  • Constructs the full hypotheses for each source sentence by walking back along the LoDTensorArray Input(ids).
  • Sort the hypotheses of each source sentence by their scores.