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()

tests/test_tokenization_reformer.py

@@ -33,6 +33,7 @@ class ReformerTokenizationTest(TokenizerTesterMixin, unittest.TestCase):
     tokenizer_class = ReformerTokenizer
     rust_tokenizer_class = ReformerTokenizerFast
     test_rust_tokenizer = True
+    test_seq2seq = False
 
     def setUp(self):
         super().setUp()

tests/test_tokenization_tapas.py

@@ -44,6 +44,7 @@ class TapasTokenizationTest(TokenizerTesterMixin, unittest.TestCase):
     test_rust_tokenizer = False
     space_between_special_tokens = True
     from_pretrained_filter = filter_non_english
+    test_seq2seq = False
 
     def get_table(
         self,

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()