# Training customization

TRL is designed with modularity in mind so that users are able to efficiently customize the training loop for their needs. Below are examples on how you can apply and test different techniques.

> [!NOTE]
> Although these examples use the [DPOTrainer](/docs/trl/v1.13.0/en/bema_for_reference_model#trl.DPOTrainer), these customization methods apply to most (if not all) trainers in TRL.

## Use different optimizers and schedulers

By default, the [DPOTrainer](/docs/trl/v1.13.0/en/bema_for_reference_model#trl.DPOTrainer) creates a `torch.optim.AdamW` optimizer. You can create and define a different optimizer and pass it to [DPOTrainer](/docs/trl/v1.13.0/en/bema_for_reference_model#trl.DPOTrainer) as follows:

```python
from datasets import load_dataset
from torch import optim
from transformers import AutoModelForCausalLM
from trl import DPOTrainer

dataset = load_dataset("trl-lib/ultrafeedback_binarized", split="train")
model = AutoModelForCausalLM.from_pretrained("Qwen/Qwen2.5-0.5B-Instruct")
optimizer = optim.SGD(model.parameters(), lr=1e-6)

trainer = DPOTrainer(
    model=model,
    train_dataset=dataset,
    optimizers=(optimizer, None),
)
trainer.train()
```

### Add a learning rate scheduler

You can also add learning rate schedulers by passing both optimizer and scheduler:

```python
from torch import optim

optimizer = optim.AdamW(model.parameters(), lr=1e-6)
lr_scheduler = optim.lr_scheduler.StepLR(optimizer, step_size=30, gamma=0.1)

trainer = DPOTrainer(..., optimizers=(optimizer, lr_scheduler))
```

## Pass 8-bit reference models

Since `trl` supports all keyword arguments when loading a model from `transformers` using `from_pretrained`, you can also leverage `load_in_8bit` from `transformers` for more memory efficient fine-tuning.

Read more about 8-bit model loading in `transformers` [Load in 8bit or 4bit](https://huggingface.co/docs/transformers/en/peft).

```python
from transformers import AutoModelForCausalLM, BitsAndBytesConfig

quantization_config = BitsAndBytesConfig(load_in_8bit=True)
ref_model = AutoModelForCausalLM.from_pretrained("Qwen/Qwen2.5-0.5B-Instruct", quantization_config=quantization_config)

trainer = DPOTrainer(..., ref_model=ref_model)
```

## Add custom callbacks

You can customize the training loop by adding callbacks for logging, monitoring, or early stopping. Callbacks allow you to execute custom code at specific points during training.

```python
from transformers import TrainerCallback

class CustomLoggingCallback(TrainerCallback):
    def on_log(self, args, state, control, logs=None, **kwargs):
        if logs is not None:
            print(f"Step {state.global_step}: {logs}")

trainer = DPOTrainer(..., callbacks=[CustomLoggingCallback()])
```

## Add custom evaluation metrics

You can define custom evaluation metrics to track during training. This is useful for monitoring model performance on specific tasks.

```python
def compute_metrics(eval_preds):
    logits, labels = eval_preds
    # Add your metric computation here
    return {"custom_metric": 0.0}

training_args = DPOConfig(..., eval_strategy="steps", eval_steps=100)

trainer = DPOTrainer(..., eval_dataset=eval_dataset, compute_metrics=compute_metrics)
```

## Use mixed precision training

Mixed precision training can significantly speed up training and reduce memory usage. You can enable it by setting `bf16=True` or `fp16=True` in the training config.

```python
# Use bfloat16 precision (recommended for modern GPUs)
training_args = DPOConfig(..., bf16=True)
```

Note: Use `bf16=True` for Ampere GPUs (A100, RTX 30xx) or newer, and `fp16=True` for older GPUs.

## Use gradient accumulation

When training with limited GPU memory, gradient accumulation allows you to simulate larger batch sizes by accumulating gradients over multiple steps before updating weights.

```python
# Simulate a batch size of 32 with per_device_train_batch_size=4 and gradient_accumulation_steps=8
training_args = DPOConfig(
    ...,
    per_device_train_batch_size=4,
    gradient_accumulation_steps=8,
)
```

## Change the training objective

Subclassing a trainer is the simplest way to customize training. The loss is defined in the `compute_loss` method, so to train with a different objective, subclass the trainer and override it. Data preparation, generation, logging, and checkpointing are inherited, so the subclass holds only what actually changes.

```python
import torch

from trl import DPOTrainer
from trl.trainer.utils import selective_log_softmax

class MyDPOTrainer(DPOTrainer):
    def compute_loss(self, model, inputs, return_outputs=False, num_items_in_batch=None):
        # In this example: the hinge loss from https://huggingface.co/papers/2309.06657
        input_ids, attention_mask = inputs["input_ids"], inputs["attention_mask"]
        shift_labels, shift_completion_mask = input_ids[:, 1:], inputs["completion_mask"][:, 1:]
        logits = model(input_ids=input_ids, attention_mask=attention_mask).logits
        with torch.no_grad():
            ref_logits = self.ref_model(input_ids=input_ids, attention_mask=attention_mask).logits

        # Sum the log-probs over the completion tokens, for the policy and for the reference model
        logps = (selective_log_softmax(logits[:, :-1], shift_labels) * shift_completion_mask).sum(dim=1)
        ref_logps = (selective_log_softmax(ref_logits[:, :-1], shift_labels) * shift_completion_mask).sum(dim=1)

        chosen_logps, rejected_logps = logps.chunk(2, dim=0)  # batch is [chosen, rejected]
        ref_chosen_logps, ref_rejected_logps = ref_logps.chunk(2, dim=0)
        delta = (chosen_logps - ref_chosen_logps) - (rejected_logps - ref_rejected_logps)
        return torch.relu(1 - self.beta * delta).mean()
```

### Add parameters to the config

Subclass the config to declare the parameters your loss needs:

```python
from dataclasses import dataclass, field

from trl import DPOConfig

@dataclass
class MyDPOConfig(DPOConfig):
    my_coef: float = field(default=0.1, metadata={"help": "Coefficient of the custom term."})
```

### Change the batch format

When the loss needs inputs that the default collator doesn't produce, pass your own collator, and override `_prepare_dataset` to leave the dataset untouched when it is already in the expected format:

```python
class MyDPOTrainer(DPOTrainer):
    def _prepare_dataset(self, dataset, *args, **kwargs):
        return dataset

trainer = MyDPOTrainer(..., data_collator=my_collator)
```

### Complete example

The block-diffusion SFT example [`examples/sft_diffusion_gemma/sft_diffusion_gemma.py`](https://github.com/huggingface/trl/blob/main/examples/sft_diffusion_gemma/sft_diffusion_gemma.py) combines the three: `DiffusionGemmaSFTConfig` extends [SFTConfig](/docs/trl/v1.13.0/en/sft_trainer#trl.SFTConfig) with the canvas and corruption parameters, and `DiffusionGemmaSFTTrainer` extends [SFTTrainer](/docs/trl/v1.13.0/en/sft_trainer#trl.SFTTrainer) and replaces the autoregressive cross-entropy with a block-diffusion denoising objective. Neither requires a change to the library.

[Antidoom](https://github.com/Liquid4All/antidoom), an open-source tool from [Liquid AI](https://huggingface.co/LiquidAI), extends [DPOTrainer](/docs/trl/v1.13.0/en/bema_for_reference_model#trl.DPOTrainer) with Final Token Preference Optimization ([Antislop](https://huggingface.co/papers/2510.15061)), a preference loss over a single token position that reduces repetition loops in reasoning models.