[Tokenizer Utils Base] Make pad function more flexible (#9928)

* change tokenizer requirement

* split line

* Correct typo from list to str

* improve style

* make other function pretty as well

* add comment

* correct typo

* add new test

* pass tests for tok without padding token

* Apply suggestions from code review

tests/test_tokenization_openai.py

@@ -128,3 +128,7 @@ class OpenAIGPTTokenizationTest(TokenizerTesterMixin, unittest.TestCase):
                     max_length=max_length,
                     padding="max_length",
                 )
+
+    # tokenizer has no padding token
+    def test_padding_different_model_input_name(self):
+        pass