Fix special tokens addition in decoder

transformers/tests/tokenization_tests_commons.py

@@ -160,6 +160,26 @@ class CommonTestCases:
             self.assertEqual(tokens[0], tokenizer.eos_token_id)
             self.assertEqual(tokens[-2], tokenizer.pad_token_id)
 
+        def test_add_special_tokens(self):
+            tokenizer = self.get_tokenizer()
+            input_text, output_text = self.get_input_output_texts()
+
+            special_token = "[SPECIAL TOKEN]"
+
+            tokenizer.add_special_tokens({"cls_token": special_token})
+            encoded_special_token = tokenizer.encode(special_token, add_special_tokens=False)
+            assert len(encoded_special_token) == 1
+
+            text = " ".join([input_text, special_token, output_text])
+            encoded = tokenizer.encode(text, add_special_tokens=False)
+
+            input_encoded = tokenizer.encode(input_text, add_special_tokens=False)
+            output_encoded = tokenizer.encode(output_text, add_special_tokens=False)
+            special_token_id = tokenizer.encode(special_token, add_special_tokens=False)
+            assert encoded == input_encoded + special_token_id + output_encoded
+
+            decoded = tokenizer.decode(encoded, skip_special_tokens=True)
+            assert special_token not in decoded
 
         def test_required_methods_tokenizer(self):
             tokenizer = self.get_tokenizer()