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_transfo_xl.py

@@ -26,6 +26,7 @@ class TransfoXLTokenizationTest(TokenizerTesterMixin, unittest.TestCase):
 
     tokenizer_class = TransfoXLTokenizer
     test_rust_tokenizer = False
+    test_seq2seq = False
 
     def setUp(self):
         super().setUp()