Refactor prepare_seq2seq_batch (#9524)

* Add target contextmanager and rework prepare_seq2seq_batch

* Fix tests, treat BART and Barthez

* Add last tokenizers

* Fix test

* Set src token before calling the superclass

* Remove special behavior for T5

* Remove needless imports

* Remove needless asserts

tests/test_modeling_marian.py

@@ -508,12 +508,6 @@ class TestMarian_en_ROMANCE(MarianIntegrationTest):
     def test_batch_generation_en_ROMANCE_multi(self):
         self._assert_generated_batch_equal_expected()
 
-    def test_tokenizer_handles_empty(self):
-        normalized = self.tokenizer.normalize("")
-        self.assertIsInstance(normalized, str)
-        with self.assertRaises(ValueError):
-            self.tokenizer.prepare_seq2seq_batch([""], return_tensors="pt")
-
     @slow
     def test_pipeline(self):
         device = 0 if torch_device == "cuda" else -1