Fix remaining issues in beam score calculation (#27808)

* Fix issues in add and is_done for BeamHypotheses

* make newly added arguments optional for better compatibility

* Directly use cur_len as generated_len, add note for retrocompatibility

* update test expectation

* make cur_len represents the length of the entire sequence including the decoder prompt

* remove redundant if/else in testing

src/transformers/generation/beam_search.py

@@ -224,8 +224,8 @@ class BeamSearchScorer(BeamScorer):
         group_index: Optional[int] = 0,
         decoder_prompt_len: Optional[int] = 0,
     ) -> Dict[str, torch.Tensor]:
-        # add up to the length which the next_scores is calculated on
-        cur_len = input_ids.shape[-1] - decoder_prompt_len + 1
+        # add up to the length which the next_scores is calculated on (including decoder prompt)
+        cur_len = input_ids.shape[-1] + 1
         batch_size = len(self._beam_hyps) // self.num_beam_groups
 
         if not (batch_size == (input_ids.shape[0] // self.group_size)):
@@ -279,15 +279,11 @@ class BeamSearchScorer(BeamScorer):
                     else:
                         beam_index = None
 
-                    # skip the corner case where the very first generated token is eos_token
-                    if decoder_prompt_len == input_ids.shape[-1]:
-                        continue
-
                     self._beam_hyps[batch_group_idx].add(
                         input_ids[batch_beam_idx].clone(),
                         next_score.item(),
                         beam_indices=beam_index,
-                        decoder_prompt_len=decoder_prompt_len,
+                        generated_len=cur_len - decoder_prompt_len,
                     )
                 else:
                     # add next predicted token since it is not eos_token
@@ -308,7 +304,7 @@ class BeamSearchScorer(BeamScorer):
 
             # Check if we are done so that we can save a pad step if all(done)
             self._done[batch_group_idx] = self._done[batch_group_idx] or self._beam_hyps[batch_group_idx].is_done(
-                next_scores[batch_idx].max().item(), cur_len
+                next_scores[batch_idx].max().item(), cur_len, decoder_prompt_len
             )
 
         return UserDict(
@@ -348,7 +344,8 @@ class BeamSearchScorer(BeamScorer):
                 final_score = final_beam_scores[batch_beam_idx].item()
                 final_tokens = input_ids[batch_beam_idx]
                 beam_index = beam_indices[batch_beam_idx] if beam_indices is not None else None
-                beam_hyp.add(final_tokens, final_score, beam_indices=beam_index, decoder_prompt_len=decoder_prompt_len)
+                generated_len = final_tokens.shape[-1] - decoder_prompt_len
+                beam_hyp.add(final_tokens, final_score, beam_indices=beam_index, generated_len=generated_len)
 
         # select the best hypotheses
         sent_lengths = input_ids.new(batch_size * self.num_beam_hyps_to_keep)
@@ -560,8 +557,8 @@ class ConstrainedBeamSearchScorer(BeamScorer):
                 indicating to which beam the next tokens shall be added.
         """
 
-        # add up to the length which the next_scores is calculated on
-        cur_len = input_ids.shape[-1] - decoder_prompt_len + 1
+        # add up to the length which the next_scores is calculated on (including decoder prompt)
+        cur_len = input_ids.shape[-1] + 1
         batch_size = len(self._beam_hyps)
         if not (batch_size == (input_ids.shape[0] // self.group_size)):
             if self.num_beam_groups > 1:
@@ -617,16 +614,11 @@ class ConstrainedBeamSearchScorer(BeamScorer):
                         else:
                             beam_index = None
 
-                        # skip the corner case where the only constraint token is
-                        # eos_token and the very first generated token is eos_token
-                        if decoder_prompt_len == input_ids.shape[-1]:
-                            continue
-
                         beam_hyp.add(
                             input_ids[batch_beam_idx].clone(),
                             next_score.item(),
                             beam_indices=beam_index,
-                            decoder_prompt_len=decoder_prompt_len,
+                            generated_len=cur_len - decoder_prompt_len,
                         )
                 else:
                     # add next predicted token since it is not eos_token
@@ -660,7 +652,7 @@ class ConstrainedBeamSearchScorer(BeamScorer):
 
             # Check if we are done so that we can save a pad step if all(done)
             self._done[batch_idx] = self._done[batch_idx] or beam_hyp.is_done(
-                next_scores[batch_idx].max().item(), cur_len
+                next_scores[batch_idx].max().item(), cur_len, decoder_prompt_len
             )
 
         return UserDict(
@@ -846,9 +838,8 @@ class ConstrainedBeamSearchScorer(BeamScorer):
                 completes_constraint = self.check_completes_constraints(final_tokens.cpu().tolist())
                 if completes_constraint:
                     beam_index = beam_indices[batch_beam_idx] if beam_indices is not None else None
-                    beam_hyp.add(
-                        final_tokens, final_score, beam_indices=beam_index, decoder_prompt_len=decoder_prompt_len
-                    )
+                    generated_len = final_tokens.shape[-1] - decoder_prompt_len
+                    beam_hyp.add(final_tokens, final_score, beam_indices=beam_index, generated_len=generated_len)
                     ids_collect.append(beam_id)
 
             # due to overly complex constraints or other factors, sometimes we can't gaurantee a successful
@@ -859,7 +850,8 @@ class ConstrainedBeamSearchScorer(BeamScorer):
                         batch_beam_idx = batch_idx * self.num_beams + beam_id
                         final_score = final_beam_scores[batch_beam_idx].item()
                         final_tokens = input_ids[batch_beam_idx]
-                        beam_hyp.add(final_tokens, final_score, decoder_prompt_len=decoder_prompt_len)
+                        generated_len = final_tokens.shape[-1] - decoder_prompt_len
+                        beam_hyp.add(final_tokens, final_score, generated_len=generated_len)
                     if len(ids_collect) >= self.num_beam_hyps_to_keep:
                         break
 
@@ -956,12 +948,17 @@ class BeamHypotheses:
         hyp: torch.LongTensor,
         sum_logprobs: float,
         beam_indices: Optional[torch.LongTensor] = None,
-        decoder_prompt_len: Optional[int] = 0,
+        generated_len: Optional[int] = None,
     ):
         """
         Add a new hypothesis to the list.
         """
-        score = sum_logprobs / ((hyp.shape[-1] - decoder_prompt_len) ** self.length_penalty)
+        if generated_len is not None:
+            score = sum_logprobs / (generated_len**self.length_penalty)
+        # This 'else' case exists for retrocompatibility
+        else:
+            score = sum_logprobs / (hyp.shape[-1] ** self.length_penalty)
+
         if len(self) < self.num_beams or score > self.worst_score:
             self.beams.append((score, hyp, beam_indices))
             if len(self) > self.num_beams:
@@ -971,7 +968,7 @@ class BeamHypotheses:
             else:
                 self.worst_score = min(score, self.worst_score)
 
-    def is_done(self, best_sum_logprobs: float, cur_len: int) -> bool:
+    def is_done(self, best_sum_logprobs: float, cur_len: int, decoder_prompt_len: Optional[int] = 0) -> bool:
         """
         If there are enough hypotheses and that none of the hypotheses being generated can become better than the worst
         one in the heap, then we are done with this sentence.
@@ -987,7 +984,7 @@ class BeamHypotheses:
         #  when `length_penalty` is positive. See the discussion below for more details.
         # https://github.com/huggingface/transformers/pull/20901#issuecomment-1369845565
         elif self.early_stopping is False:
-            highest_attainable_score = best_sum_logprobs / cur_len**self.length_penalty
+            highest_attainable_score = best_sum_logprobs / (cur_len - decoder_prompt_len) ** self.length_penalty
             ret = self.worst_score >= highest_attainable_score
             return ret
         # `"never"`: compute the best possible score, depending on the signal of `length_penalty`
@@ -996,9 +993,13 @@ class BeamHypotheses:
             # abs(`highest_attainable_score`) is obtained -> `highest_attainable_score` is negative, hence we obtain
             # its max this way
             if self.length_penalty > 0.0:
-                highest_attainable_score = best_sum_logprobs / self.max_length**self.length_penalty
+                if self.max_length <= decoder_prompt_len:
+                    raise ValueError("max_length is not larger than decoder prompt length")
+                highest_attainable_score = (
+                    best_sum_logprobs / (self.max_length - decoder_prompt_len) ** self.length_penalty
+                )
             # the opposite logic applies here (max `highest_attainable_score` from `cur_len`)
             else:
-                highest_attainable_score = best_sum_logprobs / cur_len**self.length_penalty
+                highest_attainable_score = best_sum_logprobs / (cur_len - decoder_prompt_len) ** self.length_penalty
             ret = self.worst_score >= highest_attainable_score
             return ret

tests/generation/test_framework_agnostic.py

@@ -633,11 +633,7 @@ class GenerationIntegrationTestsMixin:
             "do_sample": False,
             "num_beams": 3,
         }
-        if is_pt:
-            expectation = 20
-        else:
-            # TODO (joao): fix me
-            expectation = 13
+        expectation = 13
 
         tokenizer = AutoTokenizer.from_pretrained("hf-internal-testing/tiny-random-gpt2")
         text = """Hello, my dog is cute and"""

tests/models/vision_encoder_decoder/test_modeling_vision_encoder_decoder.py

@@ -800,7 +800,7 @@ class ViT2GPT2ModelIntegrationTest(unittest.TestCase):
 
         preds, scores = generate_step(pixel_values)
 
-        EXPECTED_SCORES = np.array([-0.64145195])
+        EXPECTED_SCORES = np.array([-0.5956343])
         max_diff = np.amax(np.abs(scores - EXPECTED_SCORES))
         self.assertLessEqual(max_diff, 1e-4)