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Fine-tuning Llama3 with Chat Data

Llama3 introduced a new prompt template for fine-tuning with chat data. In this tutorial, we’ll cover what you need to know to get you quickly started on preparing your own custom chat dataset for fine-tuning Llama3.

You will learn:

  • How the Llama3 format differs from Llama2

  • All about prompt templates and special tokens

  • How to use your own chat dataset to fine-tune Llama3

Prerequisites

Template changes from Llama2 to Llama3

The Llama2 chat model requires a specific template when prompting the pre-trained model. Since the chat model was pretrained with this prompt template, if you want to run inference on the model, you’ll need to use the same template for optimal performance on chat data. Otherwise, the model will just perform standard text completion, which may or may not align with your intended use case.

From the official Llama2 prompt template guide for the Llama2 chat model, we can see that special tags are added:

<s>[INST] <<SYS>>
You are a helpful, respectful, and honest assistant.
<</SYS>>

Hi! I am a human. [/INST] Hello there! Nice to meet you! I'm Meta AI, your friendly AI assistant </s>

Llama3 overhauled the template from Llama2 to better support multiturn conversations. The same text in the Llama3 format would look like this:

<|begin_of_text|><|start_header_id|>system<|end_header_id|>

You are a helpful, respectful, and honest assistant.<|eot_id|><|start_header_id|>user<|end_header_id|>

Hi! I am a human.<|eot_id|><|start_header_id|>assistant<|end_header_id|>

Hello there! Nice to meet you! I'm Meta AI, your friendly AI assistant<|eot_id|>

The tags are entirely different, and they are actually encoded differently than in Llama2. Let’s walk through tokenizing an example with the Llama2 template and the Llama3 template to understand how.

Tokenizing prompt templates & special tokens

Let’s say I have a sample of a single user-assistant turn accompanied with a system prompt:

sample = [
    {
        "role": "system",
        "content": "You are a helpful, respectful, and honest assistant.",
    },
    {
        "role": "user",
        "content": "Who are the most influential hip-hop artists of all time?",
    },
    {
        "role": "assistant",
        "content": "Here is a list of some of the most influential hip-hop "
        "artists of all time: 2Pac, Rakim, N.W.A., Run-D.M.C., and Nas.",
    },
]

Now, let’s format this with the Llama2ChatFormat class and see how it gets tokenized. The Llama2ChatFormat is an example of a prompt template, which simply structures a prompt with flavor text to indicate a certain task.

from torchtune.data import Llama2ChatFormat, Message

messages = [Message.from_dict(msg) for msg in sample]
formatted_messages = Llama2ChatFormat.format(messages)
print(formatted_messages)
# [
#     Message(
#         role='user',
#         content='[INST] <<SYS>>\nYou are a helpful, respectful, and honest assistant.\n<</SYS>>\n\nWho are the most influential hip-hop artists of all time? [/INST] ',
#         ...,
#     ),
#     Message(
#         role='assistant',
#         content='Here is a list of some of the most influential hip-hop artists of all time: 2Pac, Rakim, N.W.A., Run-D.M.C., and Nas.',
#         ...,
#     ),
# ]

There are also special tokens used by Llama2, which are not in the prompt template. If you look at our Llama2ChatFormat class, you’ll notice that we don’t include the <s> and </s> tokens. These are the beginning-of-sequence (BOS) and end-of-sequence (EOS) tokens that are represented differently in the tokenizer than the rest of the prompt template. Let’s tokenize this example with the SentencePieceTokenizer used by Llama2 to see why.

from torchtune.modules.tokenizers import SentencePieceTokenizer

tokenizer = SentencePieceTokenizer("/tmp/Llama-2-7b-hf/tokenizer.model")
user_message = formatted_messages[0].content
tokens = tokenizer.encode(user_message, add_bos=True, add_eos=True)
print(tokens)
# [1, 518, 25580, 29962, 3532, 14816, 29903, 6778, ..., 2]

We’ve added the BOS and EOS tokens when encoding our example text. This shows up as IDs 1 and 2. We can verify that these are our BOS and EOS tokens.

print(tokenizer.spm_model.piece_to_id("<s>"))
# 1
print(tokenizer.spm_model.piece_to_id("</s>"))
# 2

The BOS and EOS tokens are what we call special tokens, because they have their own reserved token IDs. This means that they will index to their own individual vectors in the model’s learnt embedding table. The rest of the prompt template tags, [INST] and <<SYS>> are tokenized as normal text and not their own IDs.

print(tokenizer.decode(518))
# '['
print(tokenizer.decode(25580))
# 'INST'
print(tokenizer.decode(29962))
# ']'
print(tokenizer.decode([3532, 14816, 29903, 6778]))
# '<<SYS>>'

It’s important to note that you should not place the special reserved tokens in your input prompts manually, as it will be treated as normal text and not as a special token.

print(tokenizer.encode("<s>", add_bos=False, add_eos=False))
# [529, 29879, 29958]

Now let’s take a look at Llama3’s formatting to see how it’s tokenized differently than Llama2.

from torchtune.modules.tokenizers import TikTokenTokenizer

tokenizer = TikTokenTokenizer("/tmp/Meta-Llama-3-8B/original/tokenizer.model")
messages = [Message.from_dict(msg) for msg in sample]
tokens, mask = tokenizer.tokenize_messages(messages)
print(tokenizer.decode(tokens))
# '<|start_header_id|>system<|end_header_id|>\n\nYou are a helpful, respectful,
# and honest assistant.<|eot_id|><|start_header_id|>user<|end_header_id|>\n\nWho
# are the most influential hip-hop artists of all time?<|eot_id|><|start_header_id|>
# assistant<|end_header_id|>\n\nHere is a list of some of the most influential hip-hop
# artists of all time: 2Pac, Rakim, N.W.A., Run-D.M.C., and Nas.<|eot_id|>'

Note

We used the tokenize_messages API for Llama3, which is different than encode. It simply manages adding all the special tokens in the correct places after encoding the individual messages.

We can see that the tokenizer handled all the formatting without us specifying a prompt template. It turns out that all of the additional tags are special tokens, and we don’t require a separate prompt template. We can verify this by checking if the tags get encoded as their own token IDs.

print(tokenizer._encode_special_token("<|begin_of_text|>"))
# 128000
print(tokenizer._encode_special_token("<|eot_id|>"))
# 128009

The best part is - all these special tokens are handled purely by the tokenizer. That means you won’t have to worry about messing up any required prompt templates!

When should I use a prompt template?

Whether or not to use a prompt template is governed by what your desired inference behavior is. You should use a prompt template if you are running inference on the base model and it was pre-trained with a prompt template, or you want to prime a fine-tuned model to expect a certain prompt structure on inference for a specific task.

It is not strictly necessary to fine-tune with a prompt template, but generally specific tasks will require specific templates. For example, the SummarizeTemplate provides a lightweight structure to prime your fine-tuned model for prompts asking to summarize text. This would wrap around the user message, with the assistant message untouched.

f"Summarize this dialogue:\n{dialogue}\n---\nSummary:\n"

You can fine-tune Llama2 with this template even though the model was originally pre-trained with the Llama2ChatFormat, as long as this is what the model sees during inference. The model should be robust enough to adapt to a new template.

Fine-tuning on a custom chat dataset

Let’s test our understanding by trying to fine-tune the Llama3-8B instruct model with a custom chat dataset. We’ll walk through how to set up our data so that it can be tokenized correctly and fed into our model.

Let’s say we have a local dataset saved as a CSV file that contains questions and answers from an online forum. How can we get something like this into a format Llama3 understands and tokenizes correctly?

import pandas as pd

df = pd.read_csv('your_file.csv', nrows=1)
print("Header:", df.columns.tolist())
# ['input', 'output']
print("First row:", df.iloc[0].tolist())
# [
#     "How do GPS receivers communicate with satellites?",
#     "The first thing to know is the communication is one-way...",
# ]

The Llama3 tokenizer class, TikTokenTokenizer, expects the input to be in the Message format. Let’s quickly write a function that can parse a single row from our csv file into the Message dataclass.

def message_converter(sample: Mapping[str, Any]) -> List[Message]:
    input_msg = sample["input"]
    output_msg = sample["output"]

    user_message = Message(
        role="user",
        content=input_msg,
        masked=True,  # Mask if not training on prompt
    )
    assistant_message = Message(
        role="assistant",
        content=output_msg,
        masked=False,
    )
    # A single turn conversation
    messages = [user_message, assistant_message]

    return messages

Since we’re fine-tuning Llama3, the tokenizer will handle formatting the prompt for us. But if we were fine-tuning a model that requires a template, for example the Mistral-7B model which uses the SentencePieceTokenizer, we would need to use a chat format like MistralChatFormat to format all messages according to their recommendations.

Now let’s create a builder function for our dataset that loads in our local file, converts to a list of Messages using our function, and creates a ChatDataset object.

def custom_dataset(
    *,
    tokenizer: Tokenizer,
    max_seq_len: int = 2048,  # You can expose this if you want to experiment
) -> ChatDataset:

    return ChatDataset(
        tokenizer=tokenizer,
        # For local csv files, we specify "csv" as the source, just like in
        # load_dataset
        source="csv",
        convert_to_messages=message_converter,
        # Llama3 does not need a chat format
        chat_format=None,
        max_seq_len=max_seq_len,
        # To load a local file we specify it as data_files just like in
        # load_dataset
        data_files="your_file.csv",
    )

Note

You can pass in any keyword argument for load_dataset into all our Dataset classes and they will honor them. This is useful for common parameters such as specifying the data split with split or configuration with name

Now we’re ready to start fine-tuning! We’ll use the built-in LoRA single device recipe. Use the tune cp command to get a copy of the 8B_lora_single_device.yaml config and update it to use your new dataset.

dataset:
  _component_: path.to.my.custom_dataset
  max_seq_len: 2048

Launch the fine-tune!

$ tune run lora_finetune_single_device --config custom_8B_lora_single_device.yaml epochs=15

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