🚨🚨 Fix group beam search (#24407)

* group_beam_search now works correctly

* add argument descriptions

* add a comment

* format

* make style

* change comment

* Update src/transformers/generation/beam_search.py

Co-authored-by: Joao Gante <joaofranciscocardosogante@gmail.com>

---------

Co-authored-by: shogo.fujita <shogo.fujita@legalontech.jp>
Co-authored-by: Joao Gante <joaofranciscocardosogante@gmail.com>

src/transformers/generation/beam_search.py

@@ -43,6 +43,10 @@ PROCESS_INPUTS_DOCSTRING = r"""
             The id of the *padding* token.
         eos_token_id (`Union[int, List[int]]`, *optional*):
             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.
+        group_index (`int`, *optional*):
+            The index of the group of beams. Used with [`~PreTrainedModel.group_beam_search`].
 
     Return:
         `UserDict`: A dictionary composed of the fields as defined above:
@@ -175,16 +179,22 @@ class BeamSearchScorer(BeamScorer):
         self.group_size = self.num_beams // self.num_beam_groups
 
         self._is_init = False
+        # self._beam_hyps[i*self.num_beam_groups+j] is the beam_hyps of the j-th group in the i-th mini-batch.
+        # If group_beam_search is not used, the list consists of `batch_size` beam_hyps.
         self._beam_hyps = [
             BeamHypotheses(
-                num_beams=self.num_beams,
+                num_beams=self.group_size,
                 length_penalty=self.length_penalty,
                 early_stopping=self.do_early_stopping,
                 max_length=max_length,
             )
-            for _ in range(batch_size)
+            for _ in range(batch_size * self.num_beam_groups)
         ]
-        self._done = torch.tensor([False for _ in range(batch_size)], dtype=torch.bool, device=self.device)
+        # self._done[i*self.num_beam_groups+j] indicates whether the generation of the beam_hyps of the j-th group
+        # in the i-th mini-batch is complete.
+        self._done = torch.tensor(
+            [False for _ in range(batch_size * self.num_beam_groups)], dtype=torch.bool, device=self.device
+        )
 
         if not isinstance(num_beams, int) or num_beams <= 1:
             raise ValueError(
@@ -211,9 +221,11 @@ class BeamSearchScorer(BeamScorer):
         pad_token_id: Optional[int] = None,
         eos_token_id: Optional[Union[int, List[int]]] = None,
         beam_indices: Optional[torch.LongTensor] = None,
+        group_index: 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
-        batch_size = len(self._beam_hyps)
+        batch_size = len(self._beam_hyps) // self.num_beam_groups
+
         if not (batch_size == (input_ids.shape[0] // self.group_size)):
             if self.num_beam_groups > 1:
                 raise ValueError(
@@ -234,9 +246,10 @@ class BeamSearchScorer(BeamScorer):
         if isinstance(eos_token_id, int):
             eos_token_id = [eos_token_id]
 
-        for batch_idx, beam_hyp in enumerate(self._beam_hyps):
-            if self._done[batch_idx]:
-                if self.num_beams < len(beam_hyp):
+        for batch_idx in range(batch_size):
+            batch_group_idx = batch_idx * self.num_beam_groups + group_index
+            if self._done[batch_group_idx]:
+                if self.num_beams < len(self._beam_hyps[batch_group_idx]):
                     raise ValueError(f"Batch can only be done if at least {self.num_beams} beams have been generated")
                 if eos_token_id is None or pad_token_id is None:
                     raise ValueError("Generated beams >= num_beams -> eos_token_id and pad_token have to be defined")
@@ -264,7 +277,7 @@ class BeamSearchScorer(BeamScorer):
                     else:
                         beam_index = None
 
-                    beam_hyp.add(
+                    self._beam_hyps[batch_group_idx].add(
                         input_ids[batch_beam_idx].clone(),
                         next_score.item(),
                         beam_indices=beam_index,
@@ -287,7 +300,7 @@ class BeamSearchScorer(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(
+            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
             )
 
@@ -310,20 +323,20 @@ class BeamSearchScorer(BeamScorer):
         eos_token_id: Optional[Union[int, List[int]]] = None,
         beam_indices: Optional[torch.LongTensor] = None,
     ) -> Tuple[torch.LongTensor]:
-        batch_size = len(self._beam_hyps)
+        batch_size = len(self._beam_hyps) // self.num_beam_groups
 
         if isinstance(eos_token_id, int):
             eos_token_id = [eos_token_id]
 
         # finalize all open beam hypotheses and add to generated hypotheses
-        for batch_idx, beam_hyp in enumerate(self._beam_hyps):
-            if self._done[batch_idx]:
+        for batch_group_idx, beam_hyp in enumerate(self._beam_hyps):
+            if self._done[batch_group_idx]:
                 continue
 
             # all open beam hypotheses are added to the beam hypothesis
             # beam hypothesis class automatically keeps the best beams
-            for beam_id in range(self.num_beams):
-                batch_beam_idx = batch_idx * self.num_beams + beam_id
+            for index_per_group in range(self.group_size):
+                batch_beam_idx = batch_group_idx * self.group_size + index_per_group
                 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
@@ -336,8 +349,10 @@ class BeamSearchScorer(BeamScorer):
         best_scores = torch.zeros(batch_size * self.num_beam_hyps_to_keep, device=self.device, dtype=torch.float32)
 
         # retrieve best hypotheses
-        for i, beam_hyp in enumerate(self._beam_hyps):
-            sorted_hyps = sorted(beam_hyp.beams, key=lambda x: x[0])
+        for i in range(batch_size):
+            beam_hyps_in_batch = self._beam_hyps[i * self.num_beam_groups : (i + 1) * self.num_beam_groups]
+            candidate_beams = [beam for beam_hyp in beam_hyps_in_batch for beam in beam_hyp.beams]
+            sorted_hyps = sorted(candidate_beams, key=lambda x: x[0])
             for j in range(self.num_beam_hyps_to_keep):
                 best_hyp_tuple = sorted_hyps.pop()
                 best_score = best_hyp_tuple[0]

src/transformers/generation/utils.py

@@ -3522,10 +3522,10 @@ class GenerationMixin:
             else self.generation_config.return_dict_in_generate
         )
 
-        batch_size = len(beam_scorer._beam_hyps)
         num_beams = beam_scorer.num_beams
         num_beam_groups = beam_scorer.num_beam_groups
         num_sub_beams = num_beams // num_beam_groups
+        batch_size = len(beam_scorer._beam_hyps) // num_beam_groups
         device = input_ids.device
 
         batch_beam_size, cur_len = input_ids.shape
@@ -3648,6 +3648,7 @@ class GenerationMixin:
                     pad_token_id=pad_token_id,
                     eos_token_id=eos_token_id,
                     beam_indices=process_beam_indices,
+                    group_index=beam_group_idx,
                 )
                 beam_scores[batch_group_indices] = beam_outputs["next_beam_scores"]
                 beam_next_tokens = beam_outputs["next_beam_tokens"]