# Graph classification with Transformers

Published April 14, 2023

In the previous blog, we explored some of the theoretical aspects of machine learning on graphs. This one will explore how you can do graph classification using the Transformers library. (You can also follow along by downloading the demo notebook here!)

At the moment, the only graph transformer model available in Transformers is Microsoft's Graphormer, so this is the one we will use here. We are looking forward to seeing what other models people will use and integrate 🤗

## Requirements

To follow this tutorial, you need to have installed datasets and transformers (version >= 4.27.2), which you can do with pip install -U datasets transformers.

## Data

To use graph data, you can either start from your own datasets, or use those available on the Hub. We'll focus on using already available ones, but feel free to add your datasets!

Loading a graph dataset from the Hub is very easy. Let's load the ogbg-mohiv dataset (a baseline from the Open Graph Benchmark by Stanford), stored in the OGB repository:

from datasets import load_dataset

# There is only one split on the hub

dataset = dataset.shuffle(seed=0)


This dataset already has three splits, train, validation, and test, and all these splits contain our 5 columns of interest (edge_index, edge_attr, y, num_nodes, node_feat), which you can see by doing print(dataset).

If you have other graph libraries, you can use them to plot your graphs and further inspect the dataset. For example, using PyGeometric and matplotlib:

import networkx as nx
import matplotlib.pyplot as plt

# We want to plot the first train graph
graph = dataset["train"][0]

edges = graph["edge_index"]
num_edges = len(edges[0])
num_nodes = graph["num_nodes"]

# Conversion to networkx format
G = nx.Graph()
G.add_edges_from([(edges[0][i], edges[1][i]) for i in range(num_edges)])

# Plot
nx.draw(G)


### Format

On the Hub, graph datasets are mostly stored as lists of graphs (using the jsonl format).

A single graph is a dictionary, and here is the expected format for our graph classification datasets:

• edge_index contains the indices of nodes in edges, stored as a list containing two parallel lists of edge indices.
• Type: list of 2 lists of integers.
• Example: a graph containing four nodes (0, 1, 2 and 3) and where connections are 1->2, 1->3 and 3->1 will have edge_index = [[1, 1, 3], [2, 3, 1]]. You might notice here that node 0 is not present here, as it is not part of an edge per se. This is why the next attribute is important.
• num_nodes indicates the total number of nodes available in the graph (by default, it is assumed that nodes are numbered sequentially).
• Type: integer
• Example: In our above example, num_nodes = 4.
• y maps each graph to what we want to predict from it (be it a class, a property value, or several binary label for different tasks).
• Type: list of either integers (for multi-class classification), floats (for regression), or lists of ones and zeroes (for binary multi-task classification)
• Example: We could predict the graph size (small = 0, medium = 1, big = 2). Here, y = [0].
• node_feat contains the available features (if present) for each node of the graph, ordered by node index.
• Type: list of lists of integer (Optional)
• Example: Our above nodes could have, for example, types (like different atoms in a molecule). This could give node_feat = [[1], [0], [1], [1]].
• edge_attr contains the available attributes (if present) for each edge of the graph, following the edge_index ordering.
• Type: list of lists of integers (Optional)
• Example: Our above edges could have, for example, types (like molecular bonds). This could give edge_attr = [[0], [1], [1]].

### Preprocessing

Graph transformer frameworks usually apply specific preprocessing to their datasets to generate added features and properties which help the underlying learning task (classification in our case). Here, we use Graphormer's default preprocessing, which generates in/out degree information, the shortest path between node matrices, and other properties of interest for the model.

from transformers.models.graphormer.collating_graphormer import preprocess_item, GraphormerDataCollator

dataset_processed = dataset.map(preprocess_item, batched=False)


It is also possible to apply this preprocessing on the fly, in the DataCollator's parameters (by setting on_the_fly_processing to True): not all datasets are as small as ogbg-molhiv, and for large graphs, it might be too costly to store all the preprocessed data beforehand.

## Model

Here, we load an existing pretrained model/checkpoint and fine-tune it on our downstream task, which is a binary classification task (hence num_classes = 2). We could also fine-tune our model on regression tasks (num_classes = 1) or on multi-task classification.

from transformers import GraphormerForGraphClassification

model = GraphormerForGraphClassification.from_pretrained(
"clefourrier/pcqm4mv2_graphormer_base",
num_classes=2, # num_classes for the downstream task
ignore_mismatched_sizes=True,
)


Let's look at this in more detail.

Calling the from_pretrained method on our model downloads and caches the weights for us. As the number of classes (for prediction) is dataset dependent, we pass the new num_classes as well as ignore_mismatched_sizes alongside the model_checkpoint. This makes sure a custom classification head is created, specific to our task, hence likely different from the original decoder head.

It is also possible to create a new randomly initialized model to train from scratch, either following the known parameters of a given checkpoint or by manually choosing them.

### Training or fine-tuning

To train our model simply, we will use a Trainer. To instantiate it, we will need to define the training configuration and the evaluation metric. The most important is the TrainingArguments, which is a class that contains all the attributes to customize the training. It requires a folder name, which will be used to save the checkpoints of the model.

from transformers import TrainingArguments, Trainer

training_args = TrainingArguments(
"graph-classification",
logging_dir="graph-classification",
per_device_train_batch_size=64,
per_device_eval_batch_size=64,
auto_find_batch_size=True, # batch size can be changed automatically to prevent OOMs
num_train_epochs=20,
evaluation_strategy="epoch",
logging_strategy="epoch",
push_to_hub=False,
)


For graph datasets, it is particularly important to play around with batch sizes and gradient accumulation steps to train on enough samples while avoiding out-of-memory errors.

The last argument push_to_hub allows the Trainer to push the model to the Hub regularly during training, as each saving step.

trainer = Trainer(
model=model,
args=training_args,
train_dataset=dataset_processed["train"],
eval_dataset=dataset_processed["validation"],
data_collator=GraphormerDataCollator(),
)


In the Trainer for graph classification, it is important to pass the specific data collator for the given graph dataset, which will convert individual graphs to batches for training.

train_results = trainer.train()
trainer.push_to_hub()


When the model is trained, it can be saved to the hub with all the associated training artefacts using push_to_hub.

As this model is quite big, it takes about a day to train/fine-tune for 20 epochs on CPU (IntelCore i7). To go faster, you could use powerful GPUs and parallelization instead, by launching the code either in a Colab notebook or directly on the cluster of your choice.

## Ending note

Now that you know how to use transformers to train a graph classification model, we hope you will try to share your favorite graph transformer checkpoints, models, and datasets on the Hub for the rest of the community to use!