Adding new argument max_new_tokens for generate. (#11476)

* Adding new argument `max_new_tokens` for generate.

This is a proposal to add a new argument `max_new_tokens` to `generate`.
This include a `MaxNewTokensCriteria` that enables callers that don't
know about the token length ahead (like pipelines callers) to manage
more easily the length of their generated output.

* Adding a test for the user warning when both`max_length` and
`max_new_tokens` are used together.

* Removed redundant `no_grad`.

src/transformers/generation_stopping_criteria.py

@@ -57,6 +57,29 @@ class MaxLengthCriteria(StoppingCriteria):
         return input_ids.shape[-1] >= self.max_length
 
 
+class MaxNewTokensCriteria(StoppingCriteria):
+    """
+    This class can be used to stop generation whenever the generated number of tokens exceeds :obj:`max_new_tokens`.
+    Keep in mind for decoder-only type of transformers, this will **not** include the initial prompted tokens. This is
+    very close to :obj:`MaxLengthCriteria` but ignores the number of initial tokens.
+
+    Args:
+        start_length (:obj:`int`):
+            The number of initial tokens.
+        max_new_tokens (:obj:`int`):
+            The maximum number of tokens to generate.
+    """
+
+    def __init__(self, start_length: int, max_new_tokens: int):
+        self.start_length = start_length
+        self.max_new_tokens = max_new_tokens
+        self.max_length = start_length + max_new_tokens
+
+    @add_start_docstrings(STOPPING_CRITERIA_INPUTS_DOCSTRING)
+    def __call__(self, input_ids: torch.LongTensor, scores: torch.FloatTensor, **kwargs) -> bool:
+        return input_ids.shape[-1] >= self.max_length
+
+
 class MaxTimeCriteria(StoppingCriteria):
     """
     This class can be used to stop generation whenever the full generation exceeds some amount of time. By default, the
@@ -89,6 +112,8 @@ class StoppingCriteriaList(list):
         for stopping_criterium in self:
             if isinstance(stopping_criterium, MaxLengthCriteria):
                 return stopping_criterium.max_length
+            elif isinstance(stopping_criterium, MaxNewTokensCriteria):
+                return stopping_criterium.max_length
         return None
 
 

src/transformers/generation_utils.py

@@ -42,6 +42,7 @@ from .generation_logits_process import (
 )
 from .generation_stopping_criteria import (
     MaxLengthCriteria,
+    MaxNewTokensCriteria,
     MaxTimeCriteria,
     StoppingCriteriaList,
     validate_stopping_criteria,
@@ -628,15 +629,15 @@ class GenerationMixin:
         return processors
 
     def _get_stopping_criteria(
-        self,
+        self, max_length: Optional[int], max_time: Optional[float], max_new_tokens: Optional[int], start_length: int
-        max_length: Optional[int],
-        max_time: Optional[float],
     ) -> StoppingCriteriaList:
         stopping_criteria = StoppingCriteriaList()
         if max_length is not None:
             stopping_criteria.append(MaxLengthCriteria(max_length=max_length))
         if max_time is not None:
             stopping_criteria.append(MaxTimeCriteria(max_time=max_time))
+        if max_new_tokens is not None:
+            stopping_criteria.append(MaxNewTokensCriteria(start_length=start_length, max_new_tokens=max_new_tokens))
         return stopping_criteria
 
     @torch.no_grad()
@@ -661,6 +662,7 @@ class GenerationMixin:
         encoder_no_repeat_ngram_size: Optional[int] = None,
         num_return_sequences: Optional[int] = None,
         max_time: Optional[float] = None,
+        max_new_tokens: Optional[int] = None,
         decoder_start_token_id: Optional[int] = None,
         use_cache: Optional[bool] = None,
         num_beam_groups: Optional[int] = None,
@@ -692,8 +694,11 @@ class GenerationMixin:
             input_ids (:obj:`torch.LongTensor` of shape :obj:`(batch_size, sequence_length)`, `optional`):
                 The sequence used as a prompt for the generation. If :obj:`None` the method initializes it as an empty
                 :obj:`torch.LongTensor` of shape :obj:`(1,)`.
-            max_length (:obj:`int`, `optional`, defaults to 20):
+            max_length (:obj:`int`, `optional`, defaults to :obj:`model.config.max_length`):
                 The maximum length of the sequence to be generated.
+            max_new_tokens (:obj:`int`, `optional`, defaults to None):
+                The maximum numbers of tokens to generate, ignore the current number of tokens. Use either
+                :obj:`max_new_tokens` or :obj:`max_length` but not both, they serve the same purpose.
             min_length (:obj:`int`, `optional`, defaults to 10):
                 The minimum length of the sequence to be generated.
             do_sample (:obj:`bool`, `optional`, defaults to :obj:`False`):
@@ -861,6 +866,15 @@ class GenerationMixin:
         """
 
         # set init values
+        if max_length is None and max_new_tokens is None:
+            # Both are None, default
+            max_length = self.config.max_length
+        elif max_length is not None and max_new_tokens is not None:
+            # Both are set, this is odd, raise a warning
+            warnings.warn(
+                "Both `max_length` and `max_new_tokens` have been set but they serve the same purpose.", UserWarning
+            )
+
         max_length = max_length if max_length is not None else self.config.max_length
         num_beams = num_beams if num_beams is not None else self.config.num_beams
         num_beam_groups = num_beam_groups if num_beam_groups is not None else self.config.num_beam_groups
@@ -960,7 +974,10 @@ class GenerationMixin:
             remove_invalid_values=remove_invalid_values,
         )
 
-        stopping_criteria = self._get_stopping_criteria(max_length=max_length, max_time=max_time)
+        cur_len = input_ids.shape[-1]
+        stopping_criteria = self._get_stopping_criteria(
+            max_length=max_length, max_time=max_time, max_new_tokens=max_new_tokens, start_length=cur_len
+        )
 
         if is_greedy_gen_mode:
             if num_return_sequences > 1:

tests/test_generation_stopping_criteria.py

@@ -12,6 +12,7 @@ if is_torch_available():
 
     from transformers.generation_stopping_criteria import (
         MaxLengthCriteria,
+        MaxNewTokensCriteria,
         MaxTimeCriteria,
         StoppingCriteriaList,
         validate_stopping_criteria,
@@ -58,6 +59,21 @@ class StoppingCriteriaTestCase(unittest.TestCase):
         input_ids, scores = self._get_tensors(10)
         self.assertTrue(criteria(input_ids, scores))
 
+    def test_max_new_tokens_criteria(self):
+        criteria = MaxNewTokensCriteria(start_length=5, max_new_tokens=5)
+
+        input_ids, scores = self._get_tensors(5)
+        self.assertFalse(criteria(input_ids, scores))
+
+        input_ids, scores = self._get_tensors(9)
+        self.assertFalse(criteria(input_ids, scores))
+
+        input_ids, scores = self._get_tensors(10)
+        self.assertTrue(criteria(input_ids, scores))
+
+        criteria_list = StoppingCriteriaList([criteria])
+        self.assertEqual(criteria_list.max_length, 10)
+
     def test_max_time_criteria(self):
         input_ids, scores = self._get_tensors(5)
 

tests/test_generation_utils.py

@@ -1615,3 +1615,26 @@ class GenerationIntegrationTests(unittest.TestCase):
 
         # BeamSearchScorer max_length should not influence "real" max_length
         self.assertEqual(generated_ids.tolist(), generated_ids_no_max_len.tolist())
+
+    def test_max_new_tokens(self):
+        article = """Justin Timberlake and Jessica Biel, welcome to parenthood."""
+        bart_tokenizer = BartTokenizer.from_pretrained("sshleifer/bart-tiny-random")
+        bart_model = BartForConditionalGeneration.from_pretrained("sshleifer/bart-tiny-random").to(torch_device)
+        input_ids = bart_tokenizer(article, return_tensors="pt").input_ids.to(torch_device)
+
+        self.assertEqual(list(input_ids.shape), [1, 15])
+
+        # Encoder decoder call
+        max_new_tokens = 3
+        outputs = bart_model.generate(input_ids, max_new_tokens=max_new_tokens)
+        # 1 BOS + 3 new tokens
+        self.assertEqual(list(outputs.shape), [1, 4])
+
+        # Decoder only call
+        outputs = bart_model.generate(decoder_input_ids=input_ids, max_new_tokens=max_new_tokens)
+        # 15 + 3 new tokens
+        self.assertEqual(list(outputs.shape), [1, 18])
+
+        # max_new_tokens and max_length serve the same purpose and should not be used together.
+        with self.assertWarns(UserWarning):
+            outputs = bart_model.generate(decoder_input_ids=input_ids, max_new_tokens=10, max_length=20)