DPO Trainer

TRL supports the DPO Trainer for training language models from preference data, as described in the paper Direct Preference Optimization: Your Language Model is Secretly a Reward Model by Rafailov et al., 2023. For a full example have a look at examples/scripts/dpo.py.

The first step as always is to train your SFT model, to ensure the data we train on is in-distribution for the DPO algorithm.

How DPO works

Fine-tuning a language model via DPO consists of two steps and is easier than PPO:

1. Data collection: Gather a preference dataset with positive and negative selected pairs of generation, given a prompt.
2. Optimization: Maximize the log-likelihood of the DPO loss directly.

DPO-compatible datasets can be found with the tag dpo on Hugging Face Hub.

This process is illustrated in the sketch below (from figure 1 of the original paper):

Read more about DPO algorithm in the original paper.

Expected dataset format

The DPO trainer expects a very specific format for the dataset. Since the model will be trained to directly optimize the preference of which sentence is the most relevant, given two sentences. We provide an example from the Anthropic/hh-rlhf dataset below:

Therefore the final dataset object should contain these 3 entries if you use the default DPODataCollatorWithPadding data collator. The entries should be named:

• prompt
• chosen
• rejected

for example:

dpo_dataset_dict = {
    "prompt": [
        "hello",
        "how are you",
        "What is your name?",
        "What is your name?",
        "Which is the best programming language?",
        "Which is the best programming language?",
        "Which is the best programming language?",
    ],
    "chosen": [
        "hi nice to meet you",
        "I am fine",
        "My name is Mary",
        "My name is Mary",
        "Python",
        "Python",
        "Java",
    ],
    "rejected": [
        "leave me alone",
        "I am not fine",
        "Whats it to you?",
        "I dont have a name",
        "Javascript",
        "C++",
        "C++",
    ],
}

where the prompt contains the context inputs, chosen contains the corresponding chosen responses and rejected contains the corresponding negative (rejected) responses. As can be seen a prompt can have multiple responses and this is reflected in the entries being repeated in the dictionary’s value arrays.

DPOTrainer can be used to fine-tune visual language models (VLMs). In this case, the dataset must also contain the key images, and the trainer’s tokenizer is the VLM’s processor. For example, for Idefics2, the processor expects the dataset to have the following format:

Note: Currently, VLM support is exclusive to Idefics2 and does not extend to other VLMs.

dpo_dataset_dict = {
    'images': [
        [Image.open('beach.jpg')],
        [Image.open('street.jpg')],
    ],
    'prompt': [
        'The image <image> shows',
        '<image> The image depicts',
    ],
    'chosen': [
        'a sunny beach with palm trees.',
        'a busy street with several cars and buildings.',
    ],
    'rejected': [
        'a snowy mountain with skiers.',
        'a calm countryside with green fields.',
    ],
}

Expected model format

The DPO trainer expects a model of AutoModelForCausalLM or AutoModelForVision2Seq, compared to PPO that expects AutoModelForCausalLMWithValueHead for the value function.

Using the DPOTrainer

For a detailed example have a look at the examples/scripts/dpo.py script. At a high level we need to initialize the DPOTrainer with a model we wish to train, a reference ref_model which we will use to calculate the implicit rewards of the preferred and rejected response, the beta refers to the hyperparameter of the implicit reward, and the dataset contains the 3 entries listed above. Note that the model and ref_model need to have the same architecture (ie decoder only or encoder-decoder).

training_args = DPOConfig(
    beta=0.1,
)
dpo_trainer = DPOTrainer(
    model,
    model_ref,
    args=training_args,
    train_dataset=train_dataset,
    tokenizer=tokenizer,  # for visual language models, use tokenizer=processor instead
)

After this one can then call:

dpo_trainer.train()

Note that the beta is the temperature parameter for the DPO loss, typically something in the range of 0.1 to 0.5. We ignore the reference model as beta -> 0.

Loss functions

Given the preference data, we can fit a binary classifier according to the Bradley-Terry model and in fact the DPO authors propose the sigmoid loss on the normalized likelihood via the logsigmoid to fit a logistic regression.

The RSO authors propose to use a hinge loss on the normalized likelihood from the SLiC paper. The DPOTrainer can be switched to this loss via the loss_type="hinge" argument and the beta in this case is the reciprocal of the margin.

The IPO authors provide a deeper theoretical understanding of the DPO algorithms and identify an issue with overfitting and propose an alternative loss which can be used via the loss_type="ipo" argument to the trainer. Note that the beta parameter is the reciprocal of the gap between the log-likelihood ratios of the chosen vs the rejected completion pair and thus the smaller the beta the larger this gaps is. As per the paper the loss is averaged over log-likelihoods of the completion (unlike DPO which is summed only).

The cDPO is a tweak on the DPO loss where we assume that the preference labels are noisy with some probability that can be passed to the DPOTrainer via label_smoothing argument (between 0 and 0.5) and then a conservative DPO loss is used. Pass the label_smoothing argument to the trainer to use it (default is 0).

The Robust DPO authors propose an unbiased estimate of the DPO loss that is robust to preference noise in the data. Like in cDPO, assume that the preference labels are noisy with some probability that can be passed to the DPOTrainer via label_smoothing argument (between 0 and 0.5). Use loss_type="robust" to the trainer to use it.

The EXO authors propose to minimize the reverse KL instead of the negative log-sigmoid loss of DPO which corresponds to forward KL. Setting loss_type=exo_pair and a non-zero label_smoothing (default 1e-3) leads to a simplified version of EXO on pair-wise preferences (see Eqn. (16) of the EXO paper). The full version of EXO uses K>2 completions generated by the SFT policy, which becomes an unbiased estimator of the PPO objective (up to a constant) when K is sufficiently large.

The BCO authors train a binary classifier whose logit serves as a reward so that the classifier maps {prompt, chosen completion} pairs to 1 and {prompt, rejected completion} pairs to 0. The DPOTrainer can be switched to this loss via the loss_type="bco_pair" argument.

The SPPO authors claim that SPPO is capable of solving the Nash equilibrium iteratively by pushing the chosen rewards to be as large as 1/2 and the rejected rewards to be as small as -1/2 and can alleviate data sparsity issues. The implementation using loss_type=“sppo_hard” approximates this algorithm by employing hard label probabilities, assigning 1 to the winner and 0 to the loser.

The NCA authors shows that NCA optimizes the absolute likelihood for each response rather than the relative likelihood.

The TR-DPO paper suggests syncing the reference model weights after every ref_model_sync_steps steps of SGD with weight ref_model_mixup_alpha during DPO training. To toggle this callback use the sync_ref_model flag in the DPOConfig.

The RPO paper implements an iterative preference tuning algorithm using a loss related to the RPO loss in this paper that essentially consists of the SFT loss on the chosen preferences together with a weighted DPO loss. To use this loss set the rpo_alpha in the DPOConfig to an appropriate value.

The AOT authors propose to use Distributional Preference Alignment Via Optimal Transport. Traditionally, the alignment algorithms use paired preferences at a sample level, which does not ensure alignment on the distributional level. AOT, on the other hand, can align LLMs on paired or unpaired preference data by making the reward distribution of the positive samples stochastically dominant in the first order on the distribution of negative samples. Specifically, loss_type="aot" is appropriate for paired datasets, where each prompt has both chosen and rejected responses; loss_type="aot_pair" is for unpaired datasets. In a nutshell, loss_type="aot" ensures that the log-likelihood ratio of chosen to rejected of the aligned model has higher quantiles than that ratio for the reference model. loss_type="aot_pair" ensures that the chosen reward is higher on all quantiles than the rejected reward. Note that in both cases quantiles are obtained via sorting. To fully leverage the advantages of the AOT algorithm, it is important to maximize the per-GPU batch size.

For Mixture of Experts Models: Enabling the auxiliary loss

MOEs are the most efficient if the load is about equally distributed between experts.
To ensure that we train MOEs similarly during preference-tuning, it is beneficial to add the auxiliary loss from the load balancer to the final loss.

This option is enabled by setting output_router_logits=True in the model config (e.g. MixtralConfig).
To scale how much the auxiliary loss contributes to the total loss, use the hyperparameter router_aux_loss_coef=... (default: 0.001).

Logging

While training and evaluating we record the following reward metrics:

• rewards/chosen: the mean difference between the log probabilities of the policy model and the reference model for the chosen responses scaled by beta
• rewards/rejected: the mean difference between the log probabilities of the policy model and the reference model for the rejected responses scaled by beta
• rewards/accuracies: mean of how often the chosen rewards are > than the corresponding rejected rewards
• rewards/margins: the mean difference between the chosen and corresponding rejected rewards

Accelerate DPO fine-tuning using unsloth

You can further accelerate QLoRA / LoRA (2x faster, 60% less memory) using the unsloth library that is fully compatible with SFTTrainer. Currently unsloth supports only Llama (Yi, TinyLlama, Qwen, Deepseek etc) and Mistral architectures. Some benchmarks for DPO listed below:

First install unsloth according to the official documentation. Once installed, you can incorporate unsloth into your workflow in a very simple manner; instead of loading AutoModelForCausalLM, you just need to load a FastLanguageModel as follows:

import torch
from trl import DPOConfig, DPOTrainer
from unsloth import FastLanguageModel

max_seq_length = 2048 # Supports automatic RoPE Scaling, so choose any number.

# Load model
model, tokenizer = FastLanguageModel.from_pretrained(
    model_name = "unsloth/zephyr-sft",
    max_seq_length = max_seq_length,
    dtype = None, # None for auto detection. Float16 for Tesla T4, V100, Bfloat16 for Ampere+
    load_in_4bit = True, # Use 4bit quantization to reduce memory usage. Can be False.
    # token = "hf_...", # use one if using gated models like meta-llama/Llama-2-7b-hf
)

# Do model patching and add fast LoRA weights
model = FastLanguageModel.get_peft_model(
    model,
    r = 16,
    target_modules = ["q_proj", "k_proj", "v_proj", "o_proj",
                      "gate_proj", "up_proj", "down_proj",],
    lora_alpha = 16,
    lora_dropout = 0, # Dropout = 0 is currently optimized
    bias = "none",    # Bias = "none" is currently optimized
    use_gradient_checkpointing = True,
    random_state = 3407,
)

training_args = DPOConfig(
    output_dir="./output",
    beta=0.1,
)

dpo_trainer = DPOTrainer(
    model,
    ref_model=None,
    args=training_args,
    train_dataset=train_dataset,
    tokenizer=tokenizer,
)
dpo_trainer.train()

The saved model is fully compatible with Hugging Face’s transformers library. Learn more about unsloth in their official repository.

Reference model considerations with PEFT

You have three main options (plus several variants) for how the reference model works when using PEFT, assuming the model that you would like to further enhance with DPO was tuned using (Q)LoRA.

1. Simply create two instances of the model, each loading your adapter - works fine but is very inefficient.
2. Merge the adapter into the base model, create another adapter on top, then leave the model_ref param null, in which case DPOTrainer will unload the adapter for reference inference - efficient, but has potential downsides discussed below.
3. Load the adapter twice with different names, then use set_adapter during training to swap between the adapter being DPO’d and the reference adapter - slightly less efficient compared to 2 (~adapter size VRAM overhead), but avoids the pitfalls.

Downsides to merging QLoRA before DPO (approach 2)

As suggested by Benjamin Marie, the best option for merging QLoRA adapters is to first dequantize the base model, then merge the adapter. Something similar to this script.

However, after using this approach, you will have an unquantized base model. Therefore, to use QLoRA for DPO, you will need to re-quantize the merged model or use the unquantized merge (resulting in higher memory demand).

Using option 3 - load the adapter twice

To avoid the downsides with option 2, you can load your fine-tuned adapter into the model twice, with different names, and set the model/ref adapter names in DPOTrainer.

For example:

# Load the base model.
bnb_config = BitsAndBytesConfig(
    load_in_4bit=True,
    llm_int8_threshold=6.0,
    llm_int8_has_fp16_weight=False,
    bnb_4bit_compute_dtype=torch.bfloat16,
    bnb_4bit_use_double_quant=True,
    bnb_4bit_quant_type="nf4",
)
model = AutoModelForCausalLM.from_pretrained(
    "mistralai/mixtral-8x7b-v0.1",
    load_in_4bit=True,
    quantization_config=bnb_config,
    attn_implementation="flash_attention_2",
    torch_dtype=torch.bfloat16,
    device_map="auto",
)
model.config.use_cache = False

# Load the adapter.
model = PeftModel.from_pretrained(
    model,
    "/path/to/peft",
    is_trainable=True,
    adapter_name="train",
)
# Load the adapter a second time, with a different name, which will be our reference model.
model.load_adapter("/path/to/peft", adapter_name="reference")

# Initialize the trainer, without a ref_model param.
training_args = DPOConfig(
    model_adapter_name="train",
    ref_adapter_name="reference",
)
dpo_trainer = DPOTrainer(
    model,
    args=training_args,
    ...
)

DPOTrainer

DPOConfig