Generate: contrastive search with full optional outputs (#19963)

* Use beam search functionality; Add extra outputs and test

* Add full tests for contrastive search

* Add error message on unconventional cache format

tests/models/gpt2/test_modeling_gpt2.py

@@ -763,3 +763,37 @@ class GPT2ModelLanguageGenerationTest(unittest.TestCase):
         model.generate(input_ids, do_sample=False, max_time=None, max_length=256)
         duration = datetime.datetime.now() - start
         self.assertGreater(duration, datetime.timedelta(seconds=1.5 * MAX_TIME))
+
+    @slow
+    def test_contrastive_search_gpt2(self):
+        article = (
+            "DeepMind Technologies is a British artificial intelligence subsidiary of Alphabet Inc. and research "
+            "laboratory founded in 2010. DeepMind was acquired by Google in 2014. The company is based"
+        )
+
+        gpt2_tokenizer = GPT2Tokenizer.from_pretrained("gpt2-large")
+        gpt2_model = GPT2LMHeadModel.from_pretrained("gpt2-large").to(torch_device)
+        input_ids = gpt2_tokenizer(article, return_tensors="pt").input_ids.to(torch_device)
+
+        outputs = gpt2_model.generate(input_ids, penalty_alpha=0.6, top_k=4, max_length=256)
+
+        generated_text = gpt2_tokenizer.batch_decode(outputs, skip_special_tokens=True)
+
+        self.assertListEqual(
+            generated_text,
+            [
+                "DeepMind Technologies is a British artificial intelligence subsidiary of Alphabet Inc. and research "
+                "laboratory founded in 2010. DeepMind was acquired by Google in 2014. The company is based in London, "
+                "United Kingdom\n\nGoogle has a lot of data on its users and uses it to improve its products, such as "
+                "Google Now, which helps users find the information they're looking for on the web. But the company "
+                "is not the only one to collect data on its users. Facebook, for example, has its own facial "
+                "recognition technology, as well as a database of millions of photos that it uses to personalize its "
+                "News Feed.\n\nFacebook's use of data is a hot topic in the tech industry, with privacy advocates "
+                "concerned about the company's ability to keep users' information private. In a blog post last "
+                'year, Facebook CEO Mark Zuckerberg said his company would "do our best to be transparent about our '
+                'data use and how we use it."\n\n"We have made it clear that we do not sell or share your data with '
+                'third parties," Zuckerberg wrote. "If you have questions or concerns, please reach out to us at '
+                'privacy@facebook.com."\n\nGoogle declined to comment on the privacy implications of its use of data, '
+                "but said in a statement to The Associated Press that"
+            ],
+        )