pytorch迁移学习

（8）迁移学习（Transfer Learning） 
接下来将会使用ResNet进行迁移学习，完成图片分类。目前迁移学习的方式主要有两种，一种是fineturning，就是只改变pretrain网络最后一层或者几层的网络结构，对于pretrain网络的全局参数在原来的基础上进行微调；另外一种是将ConvNet当做一个特征提取器（Feature Extractor），结构方面只改变pretrain网络最后一层或者几层的网络结构，对于参数的话固定住前面没有改变部分的参数，只对后面修改过的层进行更新。 
两种方式的代码如下：

# -*- coding:utf-8 -*-
# Transfer Learning tutorial
import torch
import torch.nn as nn
from torch.autograd import Variable
import torch.optim as optim
import numpy as np
import torchvision
from torchvision import datasets, models, transforms
import matplotlib.pyplot as plt
import time
import copy
import os


data_transforms = {
    'train': transforms.Compose([
        transforms.RandomSizedCrop(224),
        transforms.RandomHorizontalFlip(),
        transforms.ToTensor(),
        transforms.Normalize([0.485, 0.456, 0.406], [0.229, 0.224, 0.225])
    ]),
    'val': transforms.Compose([
        transforms.Scale(256),
        transforms.CenterCrop(224),
        transforms.ToTensor(),
        transforms.Normalize([0.485, 0.456, 0.406], [0.229, 0.224, 0.225])
    ])
}

data_dir = './data/hymenoptera_data'
dsets = {x: datasets.ImageFolder(os.path.join(data_dir, x), data_transforms[x])
         for x in ['train', 'val']}
dset_loaders = {x: torch.utils.data.DataLoader(dsets[x], batch_size=4,
                                               shuffle=True, num_workers=4)
                for x in ['train', 'val']}
dset_sizes = {x: len(dsets[x]) for x in ['train', 'val']}
dset_classes = dsets['train'].classes
print(dset_classes)


use_gpu = torch.cuda.is_available()
print(use_gpu)

def imshow(inp, title=None):
    inp = inp.numpy().transpose(1, 2, 0)
    mean = np.array([0.485, 0.456, 0.406])
    std = np.array([0.229, 0.224, 0.225])
    inp = std * inp + mean
    plt.imshow(inp)
    if title is not None:
        plt.title(title)

inputs, classes = next(iter(dset_loaders['train']))
out = torchvision.utils.make_grid(inputs)
imshow(out, title=[dset_classes[x] for x in classes])
# plt.show()


def train_model(model, criterion, optimizer, lr_scheduler, num_epoch=25):
    since = time.time()

    best_model = model
    best_acc = 0.0

    for epoch in range(num_epoch):
        print('Epoch {}/{}'.format(epoch, num_epoch - 1))
        print('-' * 10)

        for phase in ['train', 'val']:
            if phase == 'train':
                optimizer = lr_scheduler(optimizer, epoch)
                model.train(True)
            else:
                model.train(False)

            running_loss = 0.0
            running_corrects = 0

            for data in dset_loaders[phase]:
                inputs, labels = data
                if use_gpu:
                    inputs, labels = Variable(inputs.cuda()), Variable(labels.cuda())
                else:
                    inputs, labels = Variable(inputs), Variable(labels)
                optimizer.zero_grad()
                outputs = model(inputs)
                _, preds = torch.max(outputs.data, 1)
                loss = criterion(outputs, labels)

                if phase == 'train':
                    loss.backward()
                    optimizer.step()

                running_loss += loss.data[0]
                running_corrects += torch.sum(preds == labels.data)

            epoch_loss = running_loss / dset_sizes[phase]
            epoch_acc = running_corrects / dset_sizes[phase]

            print('{} Loss: {:.4f} Acc: {:.4f}'.format(
                phase, epoch_loss, epoch_acc))

            if phase == 'val' and epoch_acc > best_acc:
                best_acc = epoch_acc
                best_model = copy.deepcopy(model)


        print()

    time_elapsed = time.time() - since
    print('Training complete in {:.0f}m {:.0f}s'.format(
        time_elapsed // 60, time_elapsed % 60))
    print("Best val Acc: {:4f}".format(best_acc))
    return best_model



def exp_lr_scheduler(optimizer, epoch, init_lr=0.001, lr_decay_epoch=7):
    lr = init_lr * (0.1 ** (epoch // lr_decay_epoch))
    if epoch % lr_decay_epoch == 0:
        print("LR is set to {}".format(lr))
    for param_group in optimizer.param_groups:
        param_group['lr'] = lr
    return optimizer


def visualize_model(model, num_images=6):
    images_so_far = 0
    fig = plt.figure()
    for i, data in enumerate(dset_loaders['val']):
        inputs, labels = data
        if use_gpu:
            inputs, labels = Variable(inputs.cuda()), Variable(labels.cuda())
        else:
            inputs, labels = Variable(inputs), Variable(labels)

        outputs = model(inputs)
        _, preds = torch.max(outputs.data, 1)

        for j in range(inputs.size()[0]):
            images_so_far += 1
            ax = plt.subplot(num_images // 2, 2, images_so_far)
            ax.axis('off')
            ax.set_title('predicted: {}'.format(dset_classes[labels.data[j]]))
            imshow(inputs.cpu().data[j])
            if images_so_far == num_images:
                return


# Finetuning the convnet
model_ft = models.resnet18(pretrained=True)
num_ftrs = model_ft.fc.in_features
model_ft.fc = nn.Linear(num_ftrs, 2)

if use_gpu:
    model_ft = model_ft.cuda()

criterion = nn.CrossEntropyLoss()
optimizer_ft = optim.SGD(model_ft.parameters(), lr=0.001, momentum=0.9)

model_ft = train_model(model_ft, criterion, optimizer_ft, exp_lr_scheduler, num_epoch=25)

visualize_model(model_ft)
plt.ioff()
plt.show()




# ConvNet as feature extractor
model_conv = models.resnet18(pretrained=True)
for param in model_conv.parameters():
    param.requires_grad = False

num_ftrs = model_conv.fc.in_features
model_conv.fc = nn.Linear(num_ftrs, 2)

if use_gpu:
    model_conv = model_conv.cuda()

criterion = nn.CrossEntropyLoss()

optimizer_conv = optim.SGD(model_conv.fc.parameters(), lr=0.001, momentum=0.9)

model_conv = train_model(model_conv, criterion, optimizer_conv,
                         exp_lr_scheduler, num_epochs=25)

visualize_model(model_conv)
plt.ioff()
plt.show()

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运行结果如下：

['ants', 'bees']
True
Epoch 0/24
----------
LR is set to 0.001
train Loss: 0.1694 Acc: 0.6311
val Loss: 0.1212 Acc: 0.7974

Epoch 1/24
----------
train Loss: 0.1318 Acc: 0.7623
val Loss: 0.0505 Acc: 0.9216

Epoch 2/24
----------
train Loss: 0.1236 Acc: 0.7992
val Loss: 0.0510 Acc: 0.9085

Epoch 3/24
----------
train Loss: 0.1451 Acc: 0.7705
val Loss: 0.0487 Acc: 0.9412

Epoch 4/24
----------
train Loss: 0.1047 Acc: 0.8525
val Loss: 0.0753 Acc: 0.9020

Epoch 5/24
----------
train Loss: 0.1324 Acc: 0.8115
val Loss: 0.0756 Acc: 0.8889

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