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`.

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)