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_common.py

@@ -83,6 +83,7 @@ class TokenizerTesterMixin:
     from_pretrained_kwargs = None
     from_pretrained_filter = None
     from_pretrained_vocab_key = "vocab_file"
+    test_seq2seq = True
 
     def setUp(self) -> None:
         # Tokenizer.filter makes it possible to filter which Tokenizer to case based on all the
@@ -1799,10 +1800,11 @@ class TokenizerTesterMixin:
 
     @require_torch
     def test_prepare_seq2seq_batch(self):
+        if not self.test_seq2seq:
+            return
+
         tokenizer = self.get_tokenizer()
 
-        if not hasattr(tokenizer, "prepare_seq2seq_batch"):
-            return
         # Longer text that will definitely require truncation.
         src_text = [
             " UN Chief Says There Is No Military Solution in Syria",