Fix GPT2 test

tests/models/gpt2/test_tokenization_gpt2.py

@@ -280,6 +280,7 @@ def test_special_tokens_mask_input_pairs_and_bos_token(self):
     @require_jinja
     def test_tokenization_for_chat(self):
         tokenizer = GPT2Tokenizer.from_pretrained(self.tmpdirname)
+        tokenizer.chat_template = '{% for message in messages %}{{ message.content }}{{ eos_token }}{% endfor %}'
         test_chats = [
             [{"role": "system", "content": "You are a helpful chatbot."}, {"role": "user", "content": "Hello!"}],
             [