Fix generation min length (#16206)

* up

* fix min lengths

src/transformers/generation_utils.py

@@ -741,7 +741,7 @@ class GenerationMixin:
                 )
         if bad_words_ids is not None:
             processors.append(NoBadWordsLogitsProcessor(bad_words_ids, eos_token_id))
-        if min_length is not None and eos_token_id is not None and min_length > -1:
+        if min_length is not None and eos_token_id is not None and min_length > 0:
             processors.append(MinLengthLogitsProcessor(min_length, eos_token_id))
         if prefix_allowed_tokens_fn is not None:
             processors.append(PrefixConstrainedLogitsProcessor(prefix_allowed_tokens_fn, num_beams // num_beam_groups))

tests/generation/test_generation_utils.py

@@ -1949,11 +1949,14 @@ class GenerationIntegrationTests(unittest.TestCase):
     def test_custom_logits_processor(self):
         bart_tokenizer = BartTokenizer.from_pretrained("sshleifer/bart-tiny-random")
         article = """Justin Timberlake and Jessica Biel, welcome to parenthood."""
-        bart_model = BartForConditionalGeneration.from_pretrained("sshleifer/bart-tiny-random").to(torch_device)
+        bart_model = BartForConditionalGeneration.from_pretrained("sshleifer/bart-tiny-random", min_length=1).to(
+            torch_device
+        )
         input_ids = bart_tokenizer(article, return_tensors="pt").input_ids.to(torch_device)
 
         logits_processor = LogitsProcessorList()
         logits_processor.append(MinLengthLogitsProcessor(min_length=10, eos_token_id=0))
+        # it should not be allowed to both define `min_length` via config and `logits_processor` list
         with self.assertRaises(ValueError):
             bart_model.generate(input_ids, logits_processor=logits_processor)