Add support to prefixes (#221)

* Add prefix

* Fixes

* Keep original scores with prefix

* Improve prefix code

* Replace 'repeat' with 'expand'

fairseq/options.py

@@ -226,6 +226,8 @@ def add_generation_args(parser):
                        help='only print final scores')
     group.add_argument('--score-reference', action='store_true',
                        help='just score the reference translation')
+    group.add_argument('--prefix-size', default=0, type=int, metavar='PS',
+                       help=('initialize generation by target prefix of given length'))
     return group
 
 

fairseq/sequence_generator.py

@@ -51,7 +51,7 @@ class SequenceGenerator(object):
         return self
 
     def generate_batched_itr(self, data_itr, beam_size=None, maxlen_a=0.0, maxlen_b=None,
-                             cuda=False, timer=None):
+                             cuda=False, timer=None, prefix_size=0):
         """Iterate over a batched dataset and yield individual translations.
 
         Args:
@@ -75,6 +75,7 @@ class SequenceGenerator(object):
                     input['src_lengths'],
                     beam_size=beam_size,
                     maxlen=int(maxlen_a*srclen + maxlen_b),
+                    prefix_tokens=s['target'][:, :prefix_size] if prefix_size > 0 else None,
                 )
             if timer is not None:
                 timer.stop(s['ntokens'])
@@ -84,16 +85,16 @@ class SequenceGenerator(object):
                 ref = utils.strip_pad(s['target'].data[i, :], self.pad)
                 yield id, src, ref, hypos[i]
 
-    def generate(self, src_tokens, src_lengths, beam_size=None, maxlen=None):
+    def generate(self, src_tokens, src_lengths, beam_size=None, maxlen=None, prefix_tokens=None):
         """Generate a batch of translations."""
         with ExitStack() as stack:
             for model in self.models:
                 if isinstance(model.decoder, FairseqIncrementalDecoder):
                     stack.enter_context(model.decoder.incremental_inference())
             with utils.maybe_no_grad():
-                return self._generate(src_tokens, src_lengths, beam_size, maxlen)
+                return self._generate(src_tokens, src_lengths, beam_size, maxlen, prefix_tokens)
 
-    def _generate(self, src_tokens, src_lengths, beam_size=None, maxlen=None):
+    def _generate(self, src_tokens, src_lengths, beam_size=None, maxlen=None, prefix_tokens=None):
         bsz, srclen = src_tokens.size()
         maxlen = min(maxlen, self.maxlen) if maxlen is not None else self.maxlen
 
@@ -268,6 +269,15 @@ class SequenceGenerator(object):
             eos_bbsz_idx = buffer('eos_bbsz_idx')
             eos_scores = buffer('eos_scores', type_of=scores)
             if step < maxlen:
+                if prefix_tokens is not None and step < prefix_tokens.size(1):
+                    probs_slice = probs.view(bsz, -1, probs.size(-1))[:, 0, :]
+                    cand_scores = probs_slice.gather(
+                        dim=1,
+                        index=prefix_tokens[:, step].view(-1, 1).data
+                    ).expand(-1, cand_size)
+                    cand_indices = prefix_tokens[:, step].view(-1, 1).expand(bsz, cand_size).data
+                    cand_beams.resize_as_(cand_indices).fill_(0)
+                else:
                     # take the best 2 x beam_size predictions. We'll choose the first
                     # beam_size of these which don't predict eos to continue with.
                     torch.topk(
@@ -277,6 +287,7 @@ class SequenceGenerator(object):
                     )
                     torch.div(cand_indices, self.vocab_size, out=cand_beams)
                     cand_indices.fmod_(self.vocab_size)
+                
             else:
                 # finalize all active hypotheses once we hit maxlen
                 # pick the hypothesis with the highest prob of EOS right now

generate.py

@@ -90,7 +90,7 @@ def main(args):
         else:
             translations = translator.generate_batched_itr(
                 t, maxlen_a=args.max_len_a, maxlen_b=args.max_len_b,
-                cuda=use_cuda, timer=gen_timer)
+                cuda=use_cuda, timer=gen_timer, prefix_size=args.prefix_size)
         wps_meter = TimeMeter()
         for sample_id, src_tokens, target_tokens, hypos in translations:
             # Process input and ground truth