Fine-tuning Llama3 with Chat Data¶
Llama3 Instruct 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 Instruct.
How the Llama3 Instruct format differs from Llama2
All about prompt templates and special tokens
How to use your own chat dataset to fine-tune Llama3 Instruct
Be familiar with configuring datasets
Know how to download Llama3 Instruct weights
Note
This tutorial requires a version of torchtune > 0.1.1
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 Instruct overhauled the template from Llama2 to better support multiturn conversations. The same text in the Llama3 Instruct 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.
Note
The Llama3 Base model uses a different prompt template than Llama3 Instruct because it has not yet been instruct tuned and the extra special tokens are untrained. If you are running inference on the Llama3 Base model without fine-tuning we recommend the base template for optimal performance. Generally, for instruct and chat data, we recommend using Llama3 Instruct with its prompt template. The rest of this tutorial assumes you are using Llama3 Instruct.
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
llama2_tokenizer()
used by Llama2 to see
why.
from torchtune.models.llama2 import llama2_tokenizer
tokenizer = llama2_tokenizer("/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.spm_model.piece_to_id("<s>"))
# 1
print(tokenizer._spm_model.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.models.llama3 import llama3_tokenizer
tokenizer = llama3_tokenizer("/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.special_tokens["<|begin_of_text|>"])
# 128000
print(tokenizer.special_tokens["<|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, Llama3Tokenizer
,
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. The function also needs to have a train_on_input parameter.
def message_converter(sample: Mapping[str, Any], train_on_input: bool) -> List[Message]:
input_msg = sample["input"]
output_msg = sample["output"]
user_message = Message(
role="user",
content=input_msg,
masked=not train_on_input, # 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 MistralTokenizer
,
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: ModelTokenizer,
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",
# Default split of "train" is required for local files
split="train",
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. Create a new folder for your project
and make sure the dataset builder and message converter are saved in that directory,
then specify it in the config.
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