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_tokenization_openai.py

@@ -31,6 +31,7 @@ class OpenAIGPTTokenizationTest(TokenizerTesterMixin, unittest.TestCase):
     tokenizer_class = OpenAIGPTTokenizer
     rust_tokenizer_class = OpenAIGPTTokenizerFast
     test_rust_tokenizer = True
+    test_seq2seq = False
 
     def setUp(self):
         super().setUp()