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Image classification using LoRA

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Image classification using LoRA

This guide demonstrates how to use LoRA, a low-rank approximation technique, to fine-tune an image classification model. By using LoRA from 🤗 PEFT, we can reduce the number of trainable parameters in the model to only 0.77% of the original.

LoRA achieves this reduction by adding low-rank “update matrices” to specific blocks of the model, such as the attention blocks. During fine-tuning, only these matrices are trained, while the original model parameters are left unchanged. At inference time, the update matrices are merged with the original model parameters to produce the final classification result.

For more information on LoRA, please refer to the original LoRA paper.

Install dependencies

Install the libraries required for model training:

!pip install transformers accelerate evaluate datasets peft -q

Check the versions of all required libraries to make sure you are up to date:

import transformers
import accelerate
import peft

print(f"Transformers version: {transformers.__version__}")
print(f"Accelerate version: {accelerate.__version__}")
print(f"PEFT version: {peft.__version__}")
"Transformers version: 4.27.4"
"Accelerate version: 0.18.0"
"PEFT version: 0.2.0"

Authenticate to share your model

To share the fine-tuned model at the end of the training with the community, authenticate using your 🤗 token. You can obtain your token from your account settings.

from huggingface_hub import notebook_login

notebook_login()

Select a model checkpoint to fine-tune

Choose a model checkpoint from any of the model architectures supported for image classification. When in doubt, refer to the image classification task guide in 🤗 Transformers documentation.

model_checkpoint = "google/vit-base-patch16-224-in21k"

Load a dataset

To keep this example’s runtime short, let’s only load the first 5000 instances from the training set of the Food-101 dataset:

from datasets import load_dataset

dataset = load_dataset("food101", split="train[:5000]")

Dataset preparation

To prepare the dataset for training and evaluation, create label2id and id2label dictionaries. These will come in handy when performing inference and for metadata information:

labels = dataset.features["label"].names
label2id, id2label = dict(), dict()
for i, label in enumerate(labels):
    label2id[label] = i
    id2label[i] = label

id2label[2]
"baklava"

Next, load the image processor of the model you’re fine-tuning:

from transformers import AutoImageProcessor

image_processor = AutoImageProcessor.from_pretrained(model_checkpoint)

The image_processor contains useful information on which size the training and evaluation images should be resized to, as well as values that should be used to normalize the pixel values. Using the image_processor, prepare transformation functions for the datasets. These functions will include data augmentation and pixel scaling:

from torchvision.transforms import (
    CenterCrop,
    Compose,
    Normalize,
    RandomHorizontalFlip,
    RandomResizedCrop,
    Resize,
    ToTensor,
)

normalize = Normalize(mean=image_processor.image_mean, std=image_processor.image_std)
train_transforms = Compose(
    [
        RandomResizedCrop(image_processor.size["height"]),
        RandomHorizontalFlip(),
        ToTensor(),
        normalize,
    ]
)

val_transforms = Compose(
    [
        Resize(image_processor.size["height"]),
        CenterCrop(image_processor.size["height"]),
        ToTensor(),
        normalize,
    ]
)


def preprocess_train(example_batch):
    """Apply train_transforms across a batch."""
    example_batch["pixel_values"] = [train_transforms(image.convert("RGB")) for image in example_batch["image"]]
    return example_batch


def preprocess_val(example_batch):
    """Apply val_transforms across a batch."""
    example_batch["pixel_values"] = [val_transforms(image.convert("RGB")) for image in example_batch["image"]]
    return example_batch

Split the dataset into training and validation sets:

splits = dataset.train_test_split(test_size=0.1)
train_ds = splits["train"]
val_ds = splits["test"]

Finally, set the transformation functions for the datasets accordingly:

train_ds.set_transform(preprocess_train)
val_ds.set_transform(preprocess_val)

Load and prepare a model

Before loading the model, let’s define a helper function to check the total number of parameters a model has, as well as how many of them are trainable.

def print_trainable_parameters(model):
    trainable_params = 0
    all_param = 0
    for _, param in model.named_parameters():
        all_param += param.numel()
        if param.requires_grad:
            trainable_params += param.numel()
    print(
        f"trainable params: {trainable_params} || all params: {all_param} || trainable%: {100 * trainable_params / all_param:.2f}"
    )

It’s important to initialize the original model correctly as it will be used as a base to create the PeftModel you’ll actually fine-tune. Specify the label2id and id2label so that AutoModelForImageClassification can append a classification head to the underlying model, adapted for this dataset. You should see the following output:

Some weights of ViTForImageClassification were not initialized from the model checkpoint at google/vit-base-patch16-224-in21k and are newly initialized: ['classifier.weight', 'classifier.bias']
from transformers import AutoModelForImageClassification, TrainingArguments, Trainer

model = AutoModelForImageClassification.from_pretrained(
    model_checkpoint,
    label2id=label2id,
    id2label=id2label,
    ignore_mismatched_sizes=True,  # provide this in case you're planning to fine-tune an already fine-tuned checkpoint
)

Before creating a PeftModel, you can check the number of trainable parameters in the original model:

print_trainable_parameters(model)
"trainable params: 85876325 || all params: 85876325 || trainable%: 100.00"

Next, use get_peft_model to wrap the base model so that “update” matrices are added to the respective places.

from peft import LoraConfig, get_peft_model

config = LoraConfig(
    r=16,
    lora_alpha=16,
    target_modules=["query", "value"],
    lora_dropout=0.1,
    bias="none",
    modules_to_save=["classifier"],
)
lora_model = get_peft_model(model, config)
print_trainable_parameters(lora_model)
"trainable params: 667493 || all params: 86466149 || trainable%: 0.77"

Let’s unpack what’s going on here. To use LoRA, you need to specify the target modules in LoraConfig so that get_peft_model() knows which modules inside our model need to be amended with LoRA matrices. In this example, we’re only interested in targeting the query and value matrices of the attention blocks of the base model. Since the parameters corresponding to these matrices are “named” “query” and “value” respectively, we specify them accordingly in the target_modules argument of LoraConfig.

We also specify modules_to_save. After wrapping the base model with get_peft_model() along with the config, we get a new model where only the LoRA parameters are trainable (so-called “update matrices”) while the pre-trained parameters are kept frozen. However, we want the classifier parameters to be trained too when fine-tuning the base model on our custom dataset. To ensure that the classifier parameters are also trained, we specify modules_to_save. This also ensures that these modules are serialized alongside the LoRA trainable parameters when using utilities like save_pretrained() and push_to_hub().

Here’s what the other parameters mean:

  • r: The dimension used by the LoRA update matrices.
  • alpha: Scaling factor.
  • bias: Specifies if the bias parameters should be trained. None denotes none of the bias parameters will be trained.

r and alpha together control the total number of final trainable parameters when using LoRA, giving you the flexibility to balance a trade-off between end performance and compute efficiency.

By looking at the number of trainable parameters, you can see how many parameters we’re actually training. Since the goal is to achieve parameter-efficient fine-tuning, you should expect to see fewer trainable parameters in the lora_model in comparison to the original model, which is indeed the case here.

Define training arguments

For model fine-tuning, use Trainer. It accepts several arguments which you can wrap using TrainingArguments.

from transformers import TrainingArguments, Trainer


model_name = model_checkpoint.split("/")[-1]
batch_size = 128

args = TrainingArguments(
    f"{model_name}-finetuned-lora-food101",
    remove_unused_columns=False,
    evaluation_strategy="epoch",
    save_strategy="epoch",
    learning_rate=5e-3,
    per_device_train_batch_size=batch_size,
    gradient_accumulation_steps=4,
    per_device_eval_batch_size=batch_size,
    fp16=True,
    num_train_epochs=5,
    logging_steps=10,
    load_best_model_at_end=True,
    metric_for_best_model="accuracy",
    push_to_hub=True,
    label_names=["labels"],
)

Compared to non-PEFT methods, you can use a larger batch size since there are fewer parameters to train. You can also set a larger learning rate than the normal (1e-5 for example).

This can potentially also reduce the need to conduct expensive hyperparameter tuning experiments.

Prepare evaluation metric

import numpy as np
import evaluate

metric = evaluate.load("accuracy")


def compute_metrics(eval_pred):
    """Computes accuracy on a batch of predictions"""
    predictions = np.argmax(eval_pred.predictions, axis=1)
    return metric.compute(predictions=predictions, references=eval_pred.label_ids)

The compute_metrics function takes a named tuple as input: predictions, which are the logits of the model as Numpy arrays, and label_ids, which are the ground-truth labels as Numpy arrays.

Define collation function

A collation function is used by Trainer to gather a batch of training and evaluation examples and prepare them in a format that is acceptable by the underlying model.

import torch


def collate_fn(examples):
    pixel_values = torch.stack([example["pixel_values"] for example in examples])
    labels = torch.tensor([example["label"] for example in examples])
    return {"pixel_values": pixel_values, "labels": labels}

Train and evaluate

Bring everything together - model, training arguments, data, collation function, etc. Then, start the training!

trainer = Trainer(
    lora_model,
    args,
    train_dataset=train_ds,
    eval_dataset=val_ds,
    tokenizer=image_processor,
    compute_metrics=compute_metrics,
    data_collator=collate_fn,
)
train_results = trainer.train()

In just a few minutes, the fine-tuned model shows 96% validation accuracy even on this small subset of the training dataset.

trainer.evaluate(val_ds)
{
    "eval_loss": 0.14475855231285095,
    "eval_accuracy": 0.96,
    "eval_runtime": 3.5725,
    "eval_samples_per_second": 139.958,
    "eval_steps_per_second": 1.12,
    "epoch": 5.0,
}

Share your model and run inference

Once the fine-tuning is done, share the LoRA parameters with the community like so:

repo_name = f"sayakpaul/{model_name}-finetuned-lora-food101"
lora_model.push_to_hub(repo_name)

When calling push_to_hub on the lora_model, only the LoRA parameters along with any modules specified in modules_to_save are saved. Take a look at the trained LoRA parameters. You’ll see that it’s only 2.6 MB! This greatly helps with portability, especially when using a very large model to fine-tune (such as BLOOM).

Next, let’s see how to load the LoRA updated parameters along with our base model for inference. When you wrap a base model with PeftModel, modifications are done in-place. To mitigate any concerns that might stem from in-place modifications, initialize the base model just like you did earlier and construct the inference model.

from peft import PeftConfig, PeftModel


config = PeftConfig.from_pretrained(repo_name)
model = AutoModelForImageClassification.from_pretrained(
    config.base_model_name_or_path,
    label2id=label2id,
    id2label=id2label,
    ignore_mismatched_sizes=True,  # provide this in case you're planning to fine-tune an already fine-tuned checkpoint
)
# Load the LoRA model
inference_model = PeftModel.from_pretrained(model, repo_name)

Let’s now fetch an example image for inference.

from PIL import Image
import requests

url = "https://huggingface.co/datasets/sayakpaul/sample-datasets/resolve/main/beignets.jpeg"
image = Image.open(requests.get(url, stream=True).raw)
image
image of beignets

First, instantiate an image_processor from the underlying model repo.

image_processor = AutoImageProcessor.from_pretrained(repo_name)

Then, prepare the example for inference.

encoding = image_processor(image.convert("RGB"), return_tensors="pt")

Finally, run inference!

with torch.no_grad():
    outputs = inference_model(**encoding)
    logits = outputs.logits

predicted_class_idx = logits.argmax(-1).item()
print("Predicted class:", inference_model.config.id2label[predicted_class_idx])
"Predicted class: beignets"

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