🚨🚨 Fix beam score calculation issue for decoder-only models (#27351)

* Fix beam score calculation issue for decoder-only models

* Update beam search test and fix code quality issue

* Fix beam_sample, group_beam_search and constrained_beam_search

* Split test for pytorch and TF, add documentation

---------

Co-authored-by: Xin Qiu <xin.qiu@sentient.ai>

src/transformers/generation/beam_search.py

@@ -222,8 +222,10 @@ def process(
         eos_token_id: Optional[Union[int, List[int]]] = None,
         beam_indices: Optional[torch.LongTensor] = None,
         group_index: Optional[int] = 0,
+        decoder_prompt_len: Optional[int] = 0,
     ) -> Dict[str, torch.Tensor]:
-        cur_len = input_ids.shape[-1] + 1  # add up to the length which the next_scores is calculated on
+        # add up to the length which the next_scores is calculated on
+        cur_len = input_ids.shape[-1] - decoder_prompt_len + 1
         batch_size = len(self._beam_hyps) // self.num_beam_groups
 
         if not (batch_size == (input_ids.shape[0] // self.group_size)):
@@ -277,10 +279,15 @@ def process(
                     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,
                     )
                 else:
                     # add next predicted token since it is not eos_token
@@ -322,6 +329,7 @@ def finalize(
         pad_token_id: Optional[int] = None,
         eos_token_id: Optional[Union[int, List[int]]] = None,
         beam_indices: Optional[torch.LongTensor] = None,
+        decoder_prompt_len: Optional[int] = 0,
     ) -> Tuple[torch.LongTensor]:
         batch_size = len(self._beam_hyps) // self.num_beam_groups
 
@@ -340,7 +348,7 @@ def finalize(
                 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)
+                beam_hyp.add(final_tokens, final_score, beam_indices=beam_index, decoder_prompt_len=decoder_prompt_len)
 
         # select the best hypotheses
         sent_lengths = input_ids.new(batch_size * self.num_beam_hyps_to_keep)
@@ -511,6 +519,7 @@ def process(
         pad_token_id: Optional[int] = None,
         eos_token_id: Optional[Union[int, List[int]]] = None,
         beam_indices: Optional[torch.LongTensor] = None,
+        decoder_prompt_len: Optional[int] = 0,
     ) -> Tuple[torch.Tensor]:
         r"""
         Args:
@@ -535,7 +544,8 @@ def process(
                 The id of the *end-of-sequence* token. Optionally, use a list to set multiple *end-of-sequence* tokens.
             beam_indices (`torch.LongTensor`, *optional*):
                 Beam indices indicating to which beam hypothesis each token correspond.
-
+            decoder_prompt_len (`int`, *optional*):
+                The length of prompt that is included in the input to decoder.
         Return:
             `UserDict`: A dictionary composed of the fields as defined above:
 
@@ -550,7 +560,8 @@ def process(
                 indicating to which beam the next tokens shall be added.
         """
 
-        cur_len = input_ids.shape[-1] + 1  # add up to the length which the next_scores is calculated on
+        # add up to the length which the next_scores is calculated on
+        cur_len = input_ids.shape[-1] - decoder_prompt_len + 1
         batch_size = len(self._beam_hyps)
         if not (batch_size == (input_ids.shape[0] // self.group_size)):
             if self.num_beam_groups > 1:
@@ -606,10 +617,16 @@ def process(
                         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,
                         )
                 else:
                     # add next predicted token since it is not eos_token
@@ -805,6 +822,7 @@ def finalize(
         pad_token_id: Optional[int] = None,
         eos_token_id: Optional[Union[int, List[int]]] = None,
         beam_indices: Optional[torch.LongTensor] = None,
+        decoder_prompt_len: Optional[int] = 0,
     ) -> Tuple[torch.LongTensor]:
         batch_size = len(self._beam_hyps)
 
@@ -828,7 +846,9 @@ def finalize(
                 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)
+                    beam_hyp.add(
+                        final_tokens, final_score, beam_indices=beam_index, decoder_prompt_len=decoder_prompt_len
+                    )
                     ids_collect.append(beam_id)
 
             # due to overly complex constraints or other factors, sometimes we can't gaurantee a successful
@@ -839,7 +859,7 @@ def finalize(
                         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)
+                        beam_hyp.add(final_tokens, final_score, decoder_prompt_len=decoder_prompt_len)
                     if len(ids_collect) >= self.num_beam_hyps_to_keep:
                         break
 
@@ -931,11 +951,17 @@ def __len__(self):
         """
         return len(self.beams)
 
-    def add(self, hyp: torch.LongTensor, sum_logprobs: float, beam_indices: Optional[torch.LongTensor] = None):
+    def add(
+        self,
+        hyp: torch.LongTensor,
+        sum_logprobs: float,
+        beam_indices: Optional[torch.LongTensor] = None,
+        decoder_prompt_len: Optional[int] = 0,
+    ):
         """
         Add a new hypothesis to the list.
         """
-        score = sum_logprobs / (hyp.shape[-1] ** self.length_penalty)
+        score = sum_logprobs / ((hyp.shape[-1] - decoder_prompt_len) ** 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:

src/transformers/generation/utils.py

@@ -3172,6 +3172,8 @@ def beam_search(
         beam_scores = beam_scores.view((batch_size * num_beams,))
 
         this_peer_finished = False  # used by synced_gpus only
+
+        decoder_prompt_len = input_ids.shape[-1]  # record the prompt length of decoder
         while True:
             if synced_gpus:
                 # Under synced_gpus the `forward` call must continue until all gpus complete their sequence.
@@ -3246,6 +3248,7 @@ def beam_search(
                 pad_token_id=pad_token_id,
                 eos_token_id=eos_token_id,
                 beam_indices=beam_indices,
+                decoder_prompt_len=decoder_prompt_len,
             )
 
             beam_scores = beam_outputs["next_beam_scores"]
@@ -3281,6 +3284,7 @@ def beam_search(
             eos_token_id=eos_token_id,
             max_length=stopping_criteria.max_length,
             beam_indices=beam_indices,
+            decoder_prompt_len=decoder_prompt_len,
         )
 
         if return_dict_in_generate:
@@ -3500,6 +3504,8 @@ def beam_sample(
         beam_scores = beam_scores.view((batch_size * num_beams,))
 
         this_peer_finished = False  # used by synced_gpus only
+
+        decoder_prompt_len = input_ids.shape[-1]  # record the prompt length of decoder
         while True:
             if synced_gpus:
                 # Under synced_gpus the `forward` call must continue until all gpus complete their sequence.
@@ -3578,6 +3584,7 @@ def beam_sample(
                 pad_token_id=pad_token_id,
                 eos_token_id=eos_token_id,
                 beam_indices=beam_indices,
+                decoder_prompt_len=decoder_prompt_len,
             )
             beam_scores = beam_outputs["next_beam_scores"]
             beam_next_tokens = beam_outputs["next_beam_tokens"]
@@ -3612,6 +3619,7 @@ def beam_sample(
             eos_token_id=eos_token_id,
             max_length=stopping_criteria.max_length,
             beam_indices=beam_indices,
+            decoder_prompt_len=decoder_prompt_len,
         )
 
         if return_dict_in_generate:
@@ -3837,6 +3845,8 @@ def group_beam_search(
         beam_scores = beam_scores.view((batch_size * num_beams,))
 
         this_peer_finished = False  # used by synced_gpus only
+
+        decoder_prompt_len = input_ids.shape[-1]  # record the prompt length of decoder
         while True:
             if synced_gpus:
                 # Under synced_gpus the `forward` call must continue until all gpus complete their sequence.
@@ -3924,6 +3934,7 @@ def group_beam_search(
                     eos_token_id=eos_token_id,
                     beam_indices=process_beam_indices,
                     group_index=beam_group_idx,
+                    decoder_prompt_len=decoder_prompt_len,
                 )
                 beam_scores[batch_group_indices] = beam_outputs["next_beam_scores"]
                 beam_next_tokens = beam_outputs["next_beam_tokens"]
@@ -3993,6 +4004,7 @@ def group_beam_search(
             eos_token_id=eos_token_id,
             max_length=stopping_criteria.max_length,
             beam_indices=final_beam_indices,
+            decoder_prompt_len=decoder_prompt_len,
         )
 
         if return_dict_in_generate:
@@ -4220,6 +4232,8 @@ def constrained_beam_search(
         beam_scores = beam_scores.view((batch_size * num_beams,))
 
         this_peer_finished = False  # used by synced_gpus only
+
+        decoder_prompt_len = input_ids.shape[-1]  # record the prompt length of decoder
         while True:
             if synced_gpus:
                 # Under synced_gpus the `forward` call must continue until all gpus complete their sequence.
@@ -4298,6 +4312,7 @@ def constrained_beam_search(
                 pad_token_id=pad_token_id,
                 eos_token_id=eos_token_id,
                 beam_indices=beam_indices,
+                decoder_prompt_len=decoder_prompt_len,
             )
             beam_scores = beam_outputs["next_beam_scores"]
             beam_next_tokens = beam_outputs["next_beam_tokens"]
@@ -4331,6 +4346,7 @@ def constrained_beam_search(
             eos_token_id=eos_token_id,
             max_length=stopping_criteria.max_length,
             beam_indices=beam_indices,
+            decoder_prompt_len=decoder_prompt_len,
         )
 
         if return_dict_in_generate:

tests/generation/test_framework_agnostic.py

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