changed stopping condition for the beam decoder (when returning all the beams) - fixed test accordingly (#965)

Author: mirkobronzi

tensor2tensor/models/transformer_test.py

@@ -206,8 +206,6 @@ def testBeamVsFast(self):
       beam_res = beam_result.eval()
       fast_res = fast_result.eval()
 
-    self.assertEqual(fast_res.shape,
-                     (BATCH_SIZE, INPUT_LENGTH + decode_length))
     self.assertAllClose(beam_res, fast_res)
 
   def testTransformerWithoutProblem(self):

tensor2tensor/utils/beam_search.py

@@ -704,7 +704,7 @@ def inner_loop(i, alive_seq, alive_log_probs, finished_seq, finished_scores,
             finished_flags, states)
 
   def _is_finished(i, unused_alive_seq, alive_log_probs, unused_finished_seq,
-                   finished_scores, finished_in_finished, unused_states):
+                   finished_scores, unused_finished_in_finished, unused_states):
     """Checking termination condition.
 
     We terminate when we decoded up to decode_length or the lowest scoring item
@@ -716,30 +716,33 @@ def _is_finished(i, unused_alive_seq, alive_log_probs, unused_finished_seq,
       alive_log_probs: probabilities of the beams. [batch_size, beam_size]
       finished_scores: scores for each of these sequences.
         [batch_size, beam_size]
-      finished_in_finished: finished bools for each of these sequences.
-        [batch_size, beam_size]
 
     Returns:
       Bool.
     """
-    if not stop_early:
-      return tf.less(i, decode_length)
     max_length_penalty = tf.pow(((5. + tf.to_float(decode_length)) / 6.), alpha)
     # The best possible score of the most likely alive sequence.
     lower_bound_alive_scores = alive_log_probs[:, 0] / max_length_penalty
 
-    # Now to compute the lowest score of a finished sequence in finished
-    # If the sequence isn't finished, we multiply it's score by 0. since
-    # scores are all -ve, taking the min will give us the score of the lowest
-    # finished item.
-    lowest_score_of_finished_in_finished = tf.reduce_min(
-        finished_scores * tf.to_float(finished_in_finished), axis=1)
-    # If none of the sequences have finished, then the min will be 0 and
-    # we have to replace it by -ve INF if it is. The score of any seq in alive
-    # will be much higher than -ve INF and the termination condition will not
-    # be met.
-    lowest_score_of_finished_in_finished += (
-        (1. - tf.to_float(tf.reduce_any(finished_in_finished, 1))) * -INF)
+    if not stop_early:
+      # by considering the min score (in the top N beams) we ensure that
+      # the decoder will keep decoding until there is at least one beam
+      # (in the top N) that can be improved (w.r.t. the alive beams).
+      # any unfinished beam will have score -INF - thus the min
+      # will always be -INF if there is at least one unfinished beam -
+      # which means the bound_is_met condition cannot be true in this case.
+      lowest_score_of_finished_in_finished = tf.reduce_min(finished_scores)
+    else:
+      # by taking the max score we only care about the the first beam;
+      # as soon as this first beam cannot be beaten from the alive beams
+      # the beam decoder can stop.
+      # similarly to the above, if the top beam is not completed, its
+      # finished_score is -INF, thus it will not activate the
+      # bound_is_met condition. (i.e., decoder will keep going on).
+      # note we need to find the max for every sequence eparately - so, we need
+      # to keep the batch dimension (see axis=1)
+      lowest_score_of_finished_in_finished = tf.reduce_max(finished_scores,
+                                                           axis=1)
 
     bound_is_met = tf.reduce_all(
         tf.greater(lowest_score_of_finished_in_finished,

tensor2tensor/utils/beam_search_test.py

@@ -129,7 +129,7 @@ def symbols_to_logits(ids):
     self.assertAllEqual([[[0, 0, 1]]], ids)
     self.assertAllClose([[0.7 * 0.6]], np.exp(probs))
 
-  def testNotGreedyBeamTwo(self):
+  def testNotGreedyBeamTwoWithStopEarly(self):
     batch_size = 1
     beam_size = 2
     vocab_size = 3
@@ -152,11 +152,51 @@ def symbols_to_logits(ids):
         decode_length,
         vocab_size,
         0.0,
-        eos_id=1)
+        eos_id=1,
+        stop_early=True)  # defaul value, but just to make this explicit
+
+    with self.test_session():
+      ids = final_ids.eval()
+      probs = final_probs.eval()
+    # given stop_early = True, the only 'assurance' is w.r.t. the first beam
+    # (i.e., other beams may not even be completed)
+    # so, we check only the first beam
+    first_beam = ids[:, 0]
+    first_probs = probs[:, 0]
+    self.assertAllEqual([[0, 2, 1]], first_beam)
+    self.assertAllClose([0.8 * 0.5], np.exp(first_probs))
+
+  def testNotGreedyBeamTwoWithoutStopEarly(self):
+    batch_size = 1
+    beam_size = 2
+    vocab_size = 3
+    decode_length = 3
+
+    initial_ids = tf.constant([0] * batch_size)  # GO
+    probabilities = tf.constant([[[0.1, 0.1, 0.8], [0.1, 0.1, 0.8]],
+                                 [[0.4, 0.5, 0.1], [0.2, 0.4, 0.4]],
+                                 [[0.05, 0.9, 0.05], [0.4, 0.4, 0.2]]])
+
+    def symbols_to_logits(ids):
+      pos = tf.shape(ids)[1]
+      logits = tf.to_float(tf.log(probabilities[pos - 1, :]))
+      return logits
+
+    final_ids, final_probs = beam_search.beam_search(
+        symbols_to_logits,
+        initial_ids,
+        beam_size,
+        decode_length,
+        vocab_size,
+        0.0,
+        eos_id=1,
+        stop_early=False)
 
     with self.test_session():
       ids = final_ids.eval()
       probs = final_probs.eval()
+    # given stop_early = False, the algorithm will return all the beams
+    # so we can test all of them here
     self.assertAllEqual([[[0, 2, 1, 0], [0, 2, 0, 1]]], ids)
     self.assertAllClose([[0.8 * 0.5, 0.8 * 0.4 * 0.9]], np.exp(probs))