fix GPT2 token's `special_tokens_mask` when used with `add_bos_token=True` (#19036)

src/transformers/models/gpt2/tokenization_gpt2.py

@@ -261,6 +261,38 @@ class GPT2Tokenizer(PreTrainedTokenizer):
 
         return output + bos_token_ids + token_ids_1
 
+    def get_special_tokens_mask(
+        self, token_ids_0: List[int], token_ids_1: Optional[List[int]] = None, already_has_special_tokens: bool = False
+    ) -> List[int]:
+        """
+        Retrieves sequence ids from a token list that has no special tokens added. This method is called when adding
+        special tokens using the tokenizer `prepare_for_model` or `encode_plus` methods.
+
+        Args:
+            token_ids_0 (`List[int]`):
+                List of IDs.
+            token_ids_1 (`List[int]`, *optional*):
+                Optional second list of IDs for sequence pairs.
+            already_has_special_tokens (`bool`, *optional*, defaults to `False`):
+                Whether or not the token list is already formatted with special tokens for the model.
+
+        Returns:
+            `List[int]`: A list of integers in the range [0, 1]: 1 for a special token, 0 for a sequence token.
+        """
+        if already_has_special_tokens:
+            return super().get_special_tokens_mask(
+                token_ids_0=token_ids_0, token_ids_1=token_ids_1, already_has_special_tokens=True
+            )
+
+        if not self.add_bos_token:
+            return super().get_special_tokens_mask(
+                token_ids_0=token_ids_0, token_ids_1=token_ids_1, already_has_special_tokens=False
+            )
+
+        if token_ids_1 is None:
+            return [1] + ([0] * len(token_ids_0))
+        return [1] + ([0] * len(token_ids_0)) + [1] + ([0] * len(token_ids_1))
+
     def _tokenize(self, text):
         """Tokenize a string."""
         bpe_tokens = []

tests/models/gpt2/test_tokenization_gpt2.py

@@ -250,3 +250,28 @@ class GPT2TokenizationTest(TokenizerTesterMixin, unittest.TestCase):
     # tokenizer has no padding token
     def test_padding_different_model_input_name(self):
         pass
+
+    def test_special_tokens_mask_input_pairs_and_bos_token(self):
+        # TODO: change to self.get_tokenizers() when the fast version is implemented
+        tokenizers = [self.get_tokenizer(do_lower_case=False, add_bos_token=True)]
+        for tokenizer in tokenizers:
+            with self.subTest(f"{tokenizer.__class__.__name__}"):
+                sequence_0 = "Encode this."
+                sequence_1 = "This one too please."
+                encoded_sequence = tokenizer.encode(sequence_0, add_special_tokens=False)
+                encoded_sequence += tokenizer.encode(sequence_1, add_special_tokens=False)
+                encoded_sequence_dict = tokenizer.encode_plus(
+                    sequence_0,
+                    sequence_1,
+                    add_special_tokens=True,
+                    return_special_tokens_mask=True,
+                )
+                encoded_sequence_w_special = encoded_sequence_dict["input_ids"]
+                special_tokens_mask = encoded_sequence_dict["special_tokens_mask"]
+                self.assertEqual(len(special_tokens_mask), len(encoded_sequence_w_special))
+
+                filtered_sequence = [
+                    (x if not special_tokens_mask[i] else None) for i, x in enumerate(encoded_sequence_w_special)
+                ]
+                filtered_sequence = [x for x in filtered_sequence if x is not None]
+                self.assertEqual(encoded_sequence, filtered_sequence)