fixing tokenization of extra_id symbols in T5Tokenizer. Related to issue 4021 (#4353)

src/transformers/tokenization_utils.py

@@ -503,6 +503,7 @@ class SpecialTokensMixin:
             if key in self.SPECIAL_TOKENS_ATTRIBUTES:
                 if key == "additional_special_tokens":
                     assert isinstance(value, (list, tuple)) and all(isinstance(t, str) for t in value)
+                    setattr(self, key, value)
                 elif isinstance(value, AddedTokenFast):
                     setattr(self, key, str(value))
                 elif isinstance(value, str):