If no chat template is set, the
default template for that model class is used instead. Let's take a look at the template for BlenderBot:
thon

from transformers import AutoTokenizer
tokenizer = AutoTokenizer.from_pretrained("facebook/blenderbot-400M-distill")
tokenizer.default_chat_template
"{% for message in messages %}{% if message['role'] == 'user' %}{{ ' ' }}{% endif %}{{ message['content'] }}{% if not loop.last %}{{ '  ' }}{% endif %}{% endfor %}{{ eos_token }}"

That's kind of intimidating.