你已经了解了如何定义神经网络,计算损失值和网络里权重的更新。
现在你也许会想应该怎么处理数据?
通常来说,当你处理图像,文本,语音或者视频数据时,你可以使用标准 python 包将数据加载成 numpy 数组格式,然后将这个数组转换成 torch.*Tensor- 对于图像,可以用 Pillow,OpenCV
- 对于语音,可以用 scipy,librosa
- 对于文本,可以直接用 Python 或 Cython 基础数据加载模块,或者用 NLTK 和 SpaCy
这提供了极大的便利,并且避免了编写“样板代码”。
对于本教程,我们将使用CIFAR10数据集,它包含十个类别:‘airplane’, ‘automobile’, ‘bird’, ‘cat’, ‘deer’, ‘dog’, ‘frog’, ‘horse’, ‘ship’, ‘truck’。CIFAR-10 中的图像尺寸为33232,也就是RGB的3层颜色通道,每层通道内的尺寸为32*32。
训练一个图像分类器
我们将按次序的做如下几步:- 使用torchvision加载并且归一化CIFAR10的训练和测试数据集
- 定义一个卷积神经网络
- 定义一个损失函数
- 在训练样本数据上训练网络
- 在测试样本数据上测试网络
torchvision 数据集的输出是范围在[0,1]之间的 PILImage,我们将他们转换成归一化范围为[-1,1]之间的张量 Tensors。
- import torch
- import torchvision
- import torchvision.transforms as transforms
输出:
- 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’)
让我们来展示其中的一些训练图片。
- Downloading https://www.cs.toronto.edu/~kriz/cifar-10-python.tar.gz to ./data/cifar-10-python.tar.gz
- Files already downloaded and verified
- 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 = dataiter.next()
# show images
imshow(torchvision.utils.make_grid(images))
# print labels
print(‘ ‘.join(‘%5s‘ % classes[labels[j]] for j in range(4)))
输出:
- cat plane ship frog
- import torch.nn as nn
- import torch.nn.functional as F
class 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)
<span class="k">def</span> <span class="nf">forward</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">x</span><span class="p">):</span>
<span class="n">x</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">pool</span><span class="p">(</span><span class="n">F</span><span class="o">.</span><span class="n">relu</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">conv1</span><span class="p">(</span><span class="n">x</span><span class="p">)))</span>
<span class="n">x</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">pool</span><span class="p">(</span><span class="n">F</span><span class="o">.</span><span class="n">relu</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">conv2</span><span class="p">(</span><span class="n">x</span><span class="p">)))</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">x</span><span class="o">.</span><span class="n">view</span><span class="p">(</span><span class="o">-</span><span class="mi">1</span><span class="p">,</span> <span class="mi">16</span> <span class="o">*</span> <span class="mi">5</span> <span class="o">*</span> <span class="mi">5</span><span class="p">)</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">F</span><span class="o">.</span><span class="n">relu</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">fc1</span><span class="p">(</span><span class="n">x</span><span class="p">))</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">F</span><span class="o">.</span><span class="n">relu</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">fc2</span><span class="p">(</span><span class="n">x</span><span class="p">))</span>
<span class="n">x</span> <span class="o">=</span> <span class="bp">self</span><span class="o">.</span><span class="n">fc3</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="k">return</span> <span class="n">x</span>
net = Net()
- import torch.optim as optim
criterion = nn.CrossEntropyLoss()
optimizer = optim.SGD(net.parameters(), lr=0.001, momentum=0.9)
- for epoch in range(2): # loop over the dataset multiple times
<span class="n">running_loss</span> <span class="o">=</span> <span class="mf">0.0</span>
<span class="k">for</span> <span class="n">i</span><span class="p">,</span> <span class="n">data</span> <span class="ow">in</span> <span class="nb">enumerate</span><span class="p">(</span><span class="n">trainloader</span><span class="p">,</span> <span class="mi">0</span><span class="p">):</span>
<span class="c1"># get the inputs</span>
<span class="n">inputs</span><span class="p">,</span> <span class="n">labels</span> <span class="o">=</span> <span class="n">data</span>
<span class="c1"># zero the parameter gradients</span>
<span class="n">optimizer</span><span class="o">.</span><span class="n">zero_grad</span><span class="p">()</span>
<span class="c1"># forward + backward + optimize</span>
<span class="n">outputs</span> <span class="o">=</span> <span class="n">net</span><span class="p">(</span><span class="n">inputs</span><span class="p">)</span>
<span class="n">loss</span> <span class="o">=</span> <span class="n">criterion</span><span class="p">(</span><span class="n">outputs</span><span class="p">,</span> <span class="n">labels</span><span class="p">)</span>
<span class="n">loss</span><span class="o">.</span><span class="n">backward</span><span class="p">()</span>
<span class="n">optimizer</span><span class="o">.</span><span class="n">step</span><span class="p">()</span>
<span class="c1"># print statistics</span>
<span class="n">running_loss</span> <span class="o">+=</span> <span class="n">loss</span><span class="o">.</span><span class="n">item</span><span class="p">()</span>
<span class="k">if</span> <span class="n">i</span> <span class="o">%</span> <span class="mi">2000</span> <span class="o">==</span> <span class="mi">1999</span><span class="p">:</span> <span class="c1"># print every 2000 mini-batches</span>
<span class="k">print</span><span class="p">(</span><span class="s1">'[</span><span class="si">%d</span><span class="s1">, </span><span class="si">%5d</span><span class="s1">] loss: </span><span class="si">%.3f</span><span class="s1">'</span> <span class="o">%</span>
<span class="p">(</span><span class="n">epoch</span> <span class="o">+</span> <span class="mi">1</span><span class="p">,</span> <span class="n">i</span> <span class="o">+</span> <span class="mi">1</span><span class="p">,</span> <span class="n">running_loss</span> <span class="o">/</span> <span class="mi">2000</span><span class="p">))</span>
<span class="n">running_loss</span> <span class="o">=</span> <span class="mf">0.0</span>
print(‘Finished Training’)
- [1, 2000] loss: 2.187
- [1, 4000] loss: 1.852
- [1, 6000] loss: 1.672
- [1, 8000] loss: 1.566
- [1, 10000] loss: 1.490
- [1, 12000] loss: 1.461
- [2, 2000] loss: 1.389
- [2, 4000] loss: 1.364
- [2, 6000] loss: 1.343
- [2, 8000] loss: 1.318
- [2, 10000] loss: 1.282
- [2, 12000] loss: 1.286
- Finished Training
我们将用神经网络的输出作为预测的类标来检查网络的预测性能,用样本的真实类标来校对。如果预测是正确的,我们将样本添加到正确预测的列表里。
好的,第一步,让我们从测试集中显示一张图像来熟悉它。
输出:
- GroundTruth: cat ship ship plane
- outputs = net(images)
输出:
- _, predicted = torch.max(outputs, 1)
print(‘Predicted: ‘, ‘ ‘.join(‘%5s‘ % classes[predicted[j]]
for j in range(4)))
- Predicted: cat ship car ship
输出:
- correct = 0
- total = 0
- with torch.nograd():
- 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))
- Accuracy of the network on the 10000 test images: 54 %
输出:
- classcorrect = 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] += 1
for i in range(10):
print(‘Accuracy of %5s : %2d %%‘ % (
classes[i], 100 * class_correct[i] / class_total[i]))
所以接下来呢?
- Accuracy of plane : 57 %
- Accuracy of car : 73 %
- Accuracy of bird : 49 %
- Accuracy of cat : 54 %
- Accuracy of deer : 18 %
- Accuracy of dog : 20 %
- Accuracy of frog : 58 %
- Accuracy of horse : 74 %
- Accuracy of ship : 70 %
- Accuracy of truck : 66 %
我们怎么在GPU上跑这些神经网络?
在GPU上训练 就像你怎么把一个张量转移到GPU上一样,你要将神经网络转到GPU上。 如果CUDA可以用,让我们首先定义下我们的设备为第一个可见的cuda设备。
输出:
- device = torch.device(“cuda:0” if torch.cuda.is_available() else “cpu”)
# Assume that we are on a CUDA machine, then this should print a CUDA device:
print(device)
- cuda:0
接着这些方法会递归地遍历所有模块,并将它们的参数和缓冲器转换为CUDA张量。
- net.to(device)
- inputs, labels = inputs.to(device), labels.to(device)
练习:尝试增加你的网络宽度(首个 nn.Conv2d 参数设定为 2,第二个nn.Conv2d参数设定为1—它们需要有相同的个数),看看会得到怎么的速度提升。
目标:
- 深度理解了PyTorch的张量和神经网络
- 训练了一个小的神经网络来分类图像
如果你想要来看到大规模加速,使用你的所有GPU,请查看:数据并行性(https://pytorch.org/tutorials/beginner/blitz/data_parallel_tutorial.html)。PyTorch 60 分钟入门教程:数据并行处理
http://pytorchchina.com/2018/12/11/optional-data-parallelism/
下载 Python 源代码:
下载 Jupyter 源代码: