TRAINING A CLASSIFIER IN PYTORCH (2024)

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Generally, when you have to deal with image, text, audio or video data, you can use standard python packages that load data into a numpy array. Then you can convert this array into atorch.*Tensor.

For images, packages such as Pillow, OpenCV are useful
For audio, packages such as scipy and librosa
For text, either raw Python or Cython based loading, or NLTK and SpaCy are useful

Specifically for vision, we have created a package calledtorchvision, that has data loaders for common datasets such as Imagenet, CIFAR10, MNIST, etc. and data transformers for images, viz.,torchvision.datasetsandtorch.utils.data.DataLoader.

This provides a huge convenience and avoids writing boilerplate code.

For this tutorial, we will use the CIFAR10 dataset. It has the classes: ‘airplane’, ‘automobile’, ‘bird’, ‘cat’, ‘deer’, ‘dog’, ‘frog’, ‘horse’, ‘ship’, ‘truck’. The images in CIFAR-10 are of size 3x32x32, i.e. 3-channel color images of 32×32 pixels in size.

cifar10

Training an image classifier

We will do the following steps in order:

Load and normalizing the CIFAR10 training and test datasets usingtorchvision
Define a Convolutional Neural Network
Define a loss function
Train the network on the training data
Test the network on the test data

1. Loading and normalizing CIFAR10

Usingtorchvision, it’s extremely easy to load CIFAR10.

import torchimport torchvisionimport torchvision.transforms as transforms

The output of torchvision datasets are PILImage images of range [0, 1]. We transform them to Tensors of normalized range [-1, 1]. .. note:

If running on Windows and you get a BrokenPipeError, try settingthe num_worker of torch.utils.data.DataLoader() to 0.

transform = transforms.Compose( [transforms.ToTensor(), transforms.Normalize((0.5, 0.5, 0.5), (0.5, 0.5, 0.5))])trainset = torchvision.datasets.CIFAR10(root='./data', train=True, download=True, transform=transform)trainloader = torch.utils.data.DataLoader(trainset, batch_size=4, shuffle=True, num_workers=2)testset = torchvision.datasets.CIFAR10(root='./data', train=False, download=True, transform=transform)testloader = torch.utils.data.DataLoader(testset, batch_size=4, shuffle=False, num_workers=2)classes = ('plane', 'car', 'bird', 'cat', 'deer', 'dog', 'frog', 'horse', 'ship', 'truck')

Out:

2. Define a Convolutional Neural Network

Copy the neural network from the Neural Networks section before and modify it to take 3-channel images (instead of 1-channel images as it was defined).

import torch.nn as nnimport torch.nn.functional as Fclass Net(nn.Module): def __init__(self): super(Net, self).__init__() self.conv1 = nn.Conv2d(3, 6, 5) self.pool = nn.MaxPool2d(2, 2) self.conv2 = nn.Conv2d(6, 16, 5) self.fc1 = nn.Linear(16 * 5 * 5, 120) self.fc2 = nn.Linear(120, 84) self.fc3 = nn.Linear(84, 10) def forward(self, x): x = self.pool(F.relu(self.conv1(x))) x = self.pool(F.relu(self.conv2(x))) x = x.view(-1, 16 * 5 * 5) x = F.relu(self.fc1(x)) x = F.relu(self.fc2(x)) x = self.fc3(x) return xnet = Net()

3. Define a Loss function and optimizer

Let’s use a Classification Cross-Entropy loss and SGD with momentum.

import torch.optim as optimcriterion = nn.CrossEntropyLoss()optimizer = optim.SGD(net.parameters(), lr=0.001, momentum=0.9)

4. Train the network

This is when things start to get interesting. We simply have to loop over our data iterator, and feed the inputs to the network and optimize.

for epoch in range(2): # loop over the dataset multiple times running_loss = 0.0 for i, data in enumerate(trainloader, 0): # get the inputs; data is a list of [inputs, labels] inputs, labels = data # zero the parameter gradients optimizer.zero_grad() # forward + backward + optimize outputs = net(inputs) loss = criterion(outputs, labels) loss.backward() optimizer.step() # print statistics running_loss += loss.item() if i % 2000 == 1999: # print every 2000 mini-batches print('[%d, %5d] loss: %.3f' % (epoch + 1, i + 1, running_loss / 2000)) running_loss = 0.0print('Finished Training')

Out:

[1, 2000] loss: 2.266[1, 4000] loss: 1.919[1, 6000] loss: 1.704[1, 8000] loss: 1.597[1, 10000] loss: 1.531[1, 12000] loss: 1.481[2, 2000] loss: 1.410[2, 4000] loss: 1.381[2, 6000] loss: 1.330[2, 8000] loss: 1.333[2, 10000] loss: 1.318[2, 12000] loss: 1.290Finished Training

Let’s quickly save our trained model:

PATH = './cifar_net.pth'torch.save(net.state_dict(), PATH)

Seeherefor more details on saving PyTorch models.

5. Test the network on the test data

We have trained the network for 2 passes over the training dataset. But we need to check if the network has learnt anything at all.

We will check this by predicting the class label that the neural network outputs, and checking it against the ground-truth. If the prediction is correct, we add the sample to the list of correct predictions.

Okay, first step. Let us display an image from the test set to get familiar.

dataiter = iter(testloader)images, labels = dataiter.next()# print imagesimshow(torchvision.utils.make_grid(images))print('GroundTruth: ', ' '.join('%5s' % classes[labels[j]] for j in range(4)))

Out:

GroundTruth: cat ship ship plane

Next, let’s load back in our saved model (note: saving and re-loading the model wasn’t necessary here, we only did it to illustrate how to do so):

net = Net()net.load_state_dict(torch.load(PATH))

Okay, now let us see what the neural network thinks these examples above are:

outputs = net(images)

The outputs are energies for the 10 classes. The higher the energy for a class, the more the network thinks that the image is of the particular class. So, let’s get the index of the highest energy:

_, predicted = torch.max(outputs, 1)print('Predicted: ', ' '.join('%5s' % classes[predicted[j]] for j in range(4)))

Out:

Predicted: cat car plane plane

The results seem pretty good.

Let us look at how the network performs on the whole dataset.

correct = 0total = 0with torch.no_grad(): for data in testloader: images, labels = data outputs = net(images) _, predicted = torch.max(outputs.data, 1) total += labels.size(0) correct += (predicted == labels).sum().item()print('Accuracy of the network on the 10000 test images: %d %%' % ( 100 * correct / total))

Out:

Accuracy of the network on the 10000 test images: 53 %

That looks way better than chance, which is 10% accuracy (randomly picking a class out of 10 classes). Seems like the network learnt something.

Hmmm, what are the classes that performed well, and the classes that did not perform well:

class_correct = list(0. for i in range(10))class_total = list(0. for i in range(10))with torch.no_grad(): for data in testloader: images, labels = data outputs = net(images) _, predicted = torch.max(outputs, 1) c = (predicted == labels).squeeze() for i in range(4): label = labels[i] class_correct[label] += c[i].item() class_total[label] += 1for i in range(10): print('Accuracy of %5s : %2d %%' % ( classes[i], 100 * class_correct[i] / class_total[i]))

Out:

Accuracy of plane : 67 %Accuracy of car : 77 %Accuracy of bird : 42 %Accuracy of cat : 29 %Accuracy of deer : 47 %Accuracy of dog : 36 %Accuracy of frog : 69 %Accuracy of horse : 53 %Accuracy of ship : 54 %Accuracy of truck : 60 %

Okay, so what next?

How do we run these neural networks on the GPU?

Training on GPU

Just like how you transfer a Tensor onto the GPU, you transfer the neural net onto the GPU.

Let’s first define our device as the first visible cuda device if we have CUDA available:

device = torch.device("cuda:0" if torch.cuda.is_available() else "cpu")# Assuming that we are on a CUDA machine, this should print a CUDA device:print(device)

Out:

cuda:0

The rest of this section assumes thatdeviceis a CUDA device.

Then these methods will recursively go over all modules and convert their parameters and buffers to CUDA tensors:

net.to(device)

Remember that you will have to send the inputs and targets at every step to the GPU too:

inputs, labels = data[0].to(device), data[1].to(device)

Why dont I notice MASSIVE speedup compared to CPU? Because your network is really small.

Exercise:Try increasing the width of your network (argument 2 of the firstnn.Conv2d, and argument 1 of the secondnn.Conv2d– they need to be the same number), see what kind of speedup you get.

Goals achieved:

Understanding PyTorch’s Tensor library and neural networks at a high level.
Train a small neural network to classify images

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TRAINING A CLASSIFIER IN PYTORCH (2024)

Training an image classifier

1. Loading and normalizing CIFAR10

2. Define a Convolutional Neural Network

3. Define a Loss function and optimizer

4. Train the network

5. Test the network on the test data

Training on GPU

References