

What are "default" templates?
Before the introduction of chat templates, chat handling was hardcoded at the model class level. For backwards 
compatibility, we have retained this class-specific handling as default templates, also set at the class level. If a
model does not have a chat template set, but there is a default template for its model class, the TextGenerationPipeline
class and methods like apply_chat_template will use the class template instead. You can find out what the default
template for your tokenizer is by checking the tokenizer.default_chat_template attribute.
This is something we do purely for backward compatibility reasons, to avoid breaking any existing workflows. Even when
the class template is appropriate for your model, we strongly recommend overriding the default template by
setting the chat_template attribute explicitly to make it clear to users that your model has been correctly configured
for chat, and to future-proof in case the default templates are ever altered or deprecated.
What template should I use?
When setting the template for a model that's already been trained for chat, you should ensure that the template
exactly matches the message formatting that the model saw during training, or else you will probably experience
performance degradation. This is true even if you're training the model further - you will probably get the best 
performance if you keep the chat tokens constant. This is very analogous to tokenization - you generally get the
best performance for inference or fine-tuning when you precisely match the tokenization used during training.
If you're training a model from scratch, or fine-tuning a base language model for chat, on the other hand,
you have a lot of freedom to choose an appropriate template! LLMs are smart enough to learn to handle lots of different
input formats. Our default template for models that don't have a class-specific template follows the 
ChatML format, and this is a good, flexible choice for many use-cases. It looks like this:
{% for message in messages %}
    {{'<|im_start|>' + message['role'] + '\n' + message['content'] + '<|im_end|>' + '\n'}}
{% endfor %}
If you like this one, here it is in one-liner form, ready to copy into your code. The one-liner also includes
handy support for generation prompts, but note that it doesn't add BOS or EOS tokens!
If your model expects those, they won't be added automatically by apply_chat_template - in other words, the
text will be tokenized with add_special_tokens=False. This is to avoid potential conflicts between the template and
the add_special_tokens logic. If your model expects special tokens, make sure to add them to the template!
python
tokenizer.chat_template = "{% if not add_generation_prompt is defined %}{% set add_generation_prompt = false %}{% endif %}{% for message in messages %}{{'<|im_start|>' + message['role'] + '\n' + message['content'] + '<|im_end|>' + '\n'}}{% endfor %}{% if add_generation_prompt %}{{ '<|im_start|>assistant\n' }}{% endif %}"
This template wraps each message in <|im_start|> and <|im_end|> tokens, and simply writes the role as a string, which
allows for flexibility in the roles you train with. The output looks like this:
text
<|im_start|>system
You are a helpful chatbot that will do its best not to say anything so stupid that people tweet about it.<|im_end|>
<|im_start|>user
How are you?<|im_end|>
<|im_start|>assistant
I'm doing great!<|im_end|>
The "user", "system" and "assistant" roles are the standard for chat, and we recommend using them when it makes sense,
particularly if you want your model to operate well with [TextGenerationPipeline]. However, you are not limited
to these roles - templating is extremely flexible, and any string can be a role.
I want to add some chat templates! How should I get started?
If you have any chat models, you should set their tokenizer.chat_template attribute and test it using
[~PreTrainedTokenizer.apply_chat_template], then push the updated tokenizer to the Hub. This applies even if you're
not the model owner - if you're using a model with an empty chat template, or one that's still using the default class
template, please open a pull request to the model repository so that this attribute can be set properly!
Once the attribute is set, that's it, you're done! tokenizer.apply_chat_template will now work correctly for that
model, which means it is also automatically supported in places like TextGenerationPipeline!
By ensuring that models have this attribute, we can make sure that the whole community gets to use the full power of
open-source models. Formatting mismatches have been haunting the field and silently harming performance for too long - 
it's time to put an end to them!
Advanced: Template writing tips
If you're unfamiliar with Jinja, we generally find that the easiest way to write a chat template is to first
write a short Python script that formats messages the way you want, and then convert that script into a template.
Remember that the template handler will receive the conversation history as a variable called messages. Each
message is a dictionary with two keys, role and content. You will be able to access messages in your template
just like you can in Python, which means you can loop over it with {% for message in messages %} or access
individual messages with, for example, {{ messages[0] }}.
You can also use the following tips to convert your code to Jinja:
For loops
For loops in Jinja look like this:
{% for message in messages %}
{{ message['content'] }}
{% endfor %}
Note that whatever's inside the {{ expression block }} will be printed to the output. You can use operators like
+ to combine strings inside expression blocks.
If statements
If statements in Jinja look like this:
{% if message['role'] == 'user' %}
{{ message['content'] }}
{% endif %}
Note how where Python uses whitespace to mark the beginnings and ends of for and if blocks, Jinja requires you
to explicitly end them with {% endfor %} and {% endif %}.
Special variables
Inside your template, you will have access to the list of messages, but you can also access several other special
variables. These include special tokens like bos_token and eos_token, as well as the add_generation_prompt
variable that we discussed above. You can also use the loop variable to access information about the current loop
iteration, for example  using {% if loop.last %} to check if the current message is the last message in the 
conversation. Here's an example that puts these ideas together to add a generation prompt at the end of the
conversation if add_generation_prompt is True:
{% if loop.last and add_generation_prompt %}
{{ bos_token + 'Assistant:\n' }}
{% endif %}
Notes on whitespace
As much as possible, we've tried to get Jinja to ignore whitespace outside of {{ expressions }}. However, be aware
that Jinja is a general-purpose templating engine, and it may treat whitespace between blocks on the same line
as significant and print it to the output. We strongly recommend checking that your template isn't printing extra
spaces where it shouldn't be before you upload it!