cifar10_tutorial.py

# -*- coding: utf-8 -*-
"""
Training a Classifier
=====================

This is it. You have seen how to define neural networks, compute loss and make
updates to the weights of the network.

Now you might be thinking,

What about data?
----------------

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 a ``torch.*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 called
``torchvision``, that has data loaders for common datasets such as
ImageNet, CIFAR10, MNIST, etc. and data transformers for images, viz.,
``torchvision.datasets`` and ``torch.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 32x32 pixels in size.

.. figure:: /_static/img/cifar10.png
   :alt: cifar10

   cifar10


Training an image classifier
----------------------------

We will do the following steps in order:

1. Load and normalize the CIFAR10 training and test datasets using
   ``torchvision``
2. Define a Convolutional Neural Network
3. Define a loss function
4. Train the network on the training data
5. Test the network on the test data

1. Load and normalize CIFAR10
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Using ``torchvision``, it’s extremely easy to load CIFAR10.
"""
import torch
import torchvision
import 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 you are running this tutorial on Windows or MacOS and encounter a
#     BrokenPipeError or RuntimeError related to multiprocessing, try setting
#     the 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))])

batch_size = 4

trainset = torchvision.datasets.CIFAR10(root='./data', train=True,
                                        download=True, transform=transform)
trainloader = torch.utils.data.DataLoader(trainset, batch_size=batch_size,
                                          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=batch_size,
                                         shuffle=False, num_workers=2)

classes = ('plane', 'car', 'bird', 'cat',
           'deer', 'dog', 'frog', 'horse', 'ship', 'truck')

########################################################################
# Let us show some of the training images, for fun.

import matplotlib.pyplot as plt
import numpy as np

# functions to show an image


def imshow(img):
    img = img / 2 + 0.5     # unnormalize
    npimg = img.numpy()
    plt.imshow(np.transpose(npimg, (1, 2, 0)))
    plt.show()


# get some random training images
dataiter = iter(trainloader)
images, labels = next(dataiter)

# show images
imshow(torchvision.utils.make_grid(images))
# print labels
print(' '.join(f'{classes[labels[j]]:5s}' for j in range(batch_size)))


########################################################################
# 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 nn
import torch.nn.functional as F


class Net(nn.Module):
    def __init__(self):
        super().__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 = torch.flatten(x, 1) # flatten all dimensions except batch
        x = F.relu(self.fc1(x))
        x = F.relu(self.fc2(x))
        x = self.fc3(x)
        return x


net = Net()

########################################################################
# 3. Define a Loss function and optimizer
# ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
# Let's use a Classification Cross-Entropy loss and SGD with momentum.

import torch.optim as optim

criterion = 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(f'[{epoch + 1}, {i + 1:5d}] loss: {running_loss / 2000:.3f}')
            running_loss = 0.0

print('Finished Training')

########################################################################
# Let's quickly save our trained model:

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

########################################################################
# See `here <https://pytorch.org/docs/stable/notes/serialization.html>`_
# for 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 = next(dataiter)

# print images
imshow(torchvision.utils.make_grid(images))
print('GroundTruth: ', ' '.join(f'{classes[labels[j]]:5s}' for j in range(4)))

########################################################################
# 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, weights_only=True))

########################################################################
# 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(f'{classes[predicted[j]]:5s}'
                              for j in range(4)))

########################################################################
# The results seem pretty good.
#
# Let us look at how the network performs on the whole dataset.

correct = 0
total = 0
# since we're not training, we don't need to calculate the gradients for our outputs
with torch.no_grad():
    for data in testloader:
        images, labels = data
        # calculate outputs by running images through the network
        outputs = net(images)
        # the class with the highest energy is what we choose as prediction
        _, predicted = torch.max(outputs, 1)
        total += labels.size(0)
        correct += (predicted == labels).sum().item()

print(f'Accuracy of the network on the 10000 test images: {100 * correct // total} %')

########################################################################
# 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:

# prepare to count predictions for each class
correct_pred = {classname: 0 for classname in classes}
total_pred = {classname: 0 for classname in classes}

# again no gradients needed
with torch.no_grad():
    for data in testloader:
        images, labels = data
        outputs = net(images)
        _, predictions = torch.max(outputs, 1)
        # collect the correct predictions for each class
        for label, prediction in zip(labels, predictions):
            if label == prediction:
                correct_pred[classes[label]] += 1
            total_pred[classes[label]] += 1


# print accuracy for each class
for classname, correct_count in correct_pred.items():
    accuracy = 100 * float(correct_count) / total_pred[classname]
    print(f'Accuracy for class: {classname:5s} is {accuracy:.1f} %')

########################################################################
# 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)

########################################################################
# The rest of this section assumes that ``device`` is a CUDA device.
#
# Then these methods will recursively go over all modules and convert their
# parameters and buffers to CUDA tensors:
#
# .. code:: python
#
#     net.to(device)
#
#
# Remember that you will have to send the inputs and targets at every step
# to the GPU too:
#
# .. code:: python
#
#         inputs, labels = data[0].to(device), data[1].to(device)
#
# Why don't 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 first ``nn.Conv2d``, and argument 1 of the second ``nn.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
#
# Training on multiple GPUs
# -------------------------
# If you want to see even more MASSIVE speedup using all of your GPUs,
# please check out :doc:`data_parallel_tutorial`.
#
# Where do I go next?
# -------------------
#
# -  :doc:`Train neural nets to play video games </intermediate/reinforcement_q_learning>`
# -  `Train a state-of-the-art ResNet network on imagenet`_
# -  `Train a face generator using Generative Adversarial Networks`_
# -  `Train a word-level language model using Recurrent LSTM networks`_
# -  `More examples`_
# -  `More tutorials`_
# -  `Discuss PyTorch on the Forums`_
# -  `Chat with other users on Slack`_
#
# .. _Train a state-of-the-art ResNet network on imagenet: https://github.com/pytorch/examples/tree/master/imagenet
# .. _Train a face generator using Generative Adversarial Networks: https://github.com/pytorch/examples/tree/master/dcgan
# .. _Train a word-level language model using Recurrent LSTM networks: https://github.com/pytorch/examples/tree/master/word_language_model
# .. _More examples: https://github.com/pytorch/examples
# .. _More tutorials: https://github.com/pytorch/tutorials
# .. _Discuss PyTorch on the Forums: https://discuss.pytorch.org/
# .. _Chat with other users on Slack: https://pytorch.slack.com/messages/beginner/

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del dataiter
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