MxNet新前端Gluon模型转换到Symbol

1. 导入各种包

from mxnet import gluon
from mxnet.gluon import nn
import matplotlib.pyplot as plt
from mxnet import autograd as autograd
from mxnet import nd
import mxnet as mx
from collections import namedtuple
import random

2. 准备数据

使用和mnist很像的FashionMNIST数据集，使用Gluon下载

def transform(data,label):
    return data.astype('float32')/255,label.astype('float32')

fashion_train = gluon.data.vision.FashionMNIST(root='./',train=True,transform=transform)
fashion_test = gluon.data.vision.FashionMNIST(root='./',train=True, transform=transform)

batch_size = 256
train_data = gluon.data.DataLoader(fashion_train,batch_size,shuffle=True)
test_data = gluon.data.DataLoader(fashion_test,batch_size,shuffle=True)

用于显示图像和标签

def show_images(images):
    n = images.shape[0]
    _, figs = plt.subplots(1, n, figsize=(15, 15))
    for i in range(n):
        figs[i].imshow(images[i].reshape((28, 28)).asnumpy())
        figs[i].axes.get_xaxis().set_visible(False)
        figs[i].axes.get_yaxis().set_visible(False)
    plt.show()

def get_text_labels(label):
    text_labels = [
        't-shirt', 'trouser', 'pullover', 'dress,', 'coat',
        'sandal', 'shirt', 'sneaker', 'bag', 'ankle boot'
    ]
    return [text_labels[int(i)] for i in label]

看下数据集长啥样

data,label = fashion_train[5:19]
show_images(data)
print(get_text_labels(label))

内容示例

['coat', 'coat', 'sandal', 'coat', 'bag', 't-shirt', 'bag', 'ankle boot', 't-shirt', 'pullover', 'pullover', 'ankle boot', 'dress,', 'dress,']

3. 精度计算函数

def accuracy(output, label):
    return nd.mean(output.argmax(axis=1)==label).asscalar()

def evaluate_accuracy(data_iterator, net):
    acc = 0.
    for data, label in data_iterator:
        output = net(nd.transpose(data,(0,3,1,2)))
        acc += accuracy(output, label)
    return acc / len(data_iterator)

4. 定义网络

4.1 自己定义的层

Gluon模型转到Symbol下只能用HybridSequential模式，HybridSequential是静态图，会对计算有优化，不过HybridSequential和Sequential可以很方便的转换，确切的就是一行代码的事。同样自定义的网络，要使用HybridBlock，和Block没有多大区别

class MyDense(nn.HybridBlock):
    def __init__(self,**kwargs):
        super(MyDense,self).__init__(**kwargs)
        with self.name_scope():
            self.dense0 = nn.Dense(256)
            self.dense1 = nn.Dense(10)
    def hybrid_forward(self,F,x):   #  这里要使用hybrid_forward而不是forward，并且多了个参数F
        return self.dense1(F.relu(self.dense0(x)))    #  F的作用就是替代 nd，如果是静态图，就是用 sym，否则使用 nd

4.2 使用自定义的层和自带的层组成完整的网络

网络定义和动态图一样，只不过把Sequential替换成了HybridSequential，在最后使用hybridize()会对静态图进行优化

net = nn.HybridSequential()
with net.name_scope():
    net.add(gluon.nn.Conv2D(channels=20, kernel_size=5, activation='relu'))
    net.add(gluon.nn.MaxPool2D(pool_size=2, strides=2))
    net.add(gluon.nn.Conv2D(channels=50, kernel_size=3, activation='relu'))
    net.add(gluon.nn.MaxPool2D(pool_size=2, strides=2))
    net.add(gluon.nn.Flatten())
    net.add(MyDense())
net.initialize(init=mx.init.Xavier())
net.hybridize()
net

HybridSequential(
  (0): Conv2D(20, kernel_size=(5, 5), stride=(1, 1))
  (1): MaxPool2D(size=(2, 2), stride=(2, 2), padding=(0, 0), ceil_mode=False)
  (2): Conv2D(50, kernel_size=(3, 3), stride=(1, 1))
  (3): MaxPool2D(size=(2, 2), stride=(2, 2), padding=(0, 0), ceil_mode=False)
  (4): Flatten
  (5): MyDense(
    (dense0): Dense(256, linear)
    (dense1): Dense(10, linear)
  )
)

5. 训练

使用Adam优化算法，训练的速度会快点

softmax_cross_entropy = gluon.loss.SoftmaxCrossEntropyLoss()
trainer = gluon.Trainer(net.collect_params(), 'Adam', {'learning_rate': 0.008})

for epoch in range(5):
    train_loss = 0.
    train_acc = 0.
    test_acc = 0.
    for data, label in train_data:
        data = nd.transpose(data,(0,3,1,2))
        with autograd.record():
            output = net(data)
            loss = softmax_cross_entropy(output, label)
        loss.backward()
        trainer.step(batch_size)

        train_loss += nd.mean(loss).asscalar()
        train_acc += accuracy(output, label)

    test_acc = evaluate_accuracy(test_data, net)
    print("Epoch %d. Loss: %f, Train acc %f, Test acc %f" % (
            epoch, train_loss/len(train_data), train_acc/len(train_data), test_acc))

Epoch 0. Loss: 0.498041, Train acc 0.817226, Test acc 0.865459
Epoch 1. Loss: 0.312128, Train acc 0.884813, Test acc 0.894265
Epoch 2. Loss: 0.274009, Train acc 0.898454, Test acc 0.898604
Epoch 3. Loss: 0.247741, Train acc 0.906521, Test acc 0.914910
Epoch 4. Loss: 0.226967, Train acc 0.913736, Test acc 0.914334

6. 保存成Symbol格式的网络和参数（重点）

要注意保存网络参数的时候，需要net.collect_params().save()这样保存，而不是net.save_params()保存
最新版的mxnet已经有可以导出到symbol格式下的接口了。需要mxnet版本在20171015以上
下面示例代码也已经改成新版的保存，加载方式

#新版本的保存方式
net.export('Gluon_FashionMNIST')

7. 使用Symbol加载网络并绑定

symnet = mx.symbol.load('Gluon_FashionMNIST-symbol.json')
mod = mx.mod.Module(symbol=symnet, context=mx.cpu())
mod.bind(data_shapes=[('data', (1, 1, 28, 28))])
mod.load_params('Gluon_FashionMNIST-0000.params')
Batch = namedtuple('Batch', ['data'])

8. 预测试试看效果

img,label = fashion_test[random.randint(0, 60000)]
data = img.transpose([2,0,1])
data = data.reshape([1,1,28,28])

mod.forward(Batch([data]))
out = mod.get_outputs()
prob = out[0]
predicted_labels = prob.argmax(axis=1)

plt.imshow(img.reshape((28, 28)).asnumpy())
plt.axis('off')
plt.show()
print('predicted labels:',get_text_labels(predicted_labels.asnumpy()))

print('true labels:',get_text_labels([label]))

预测内容

predicted labels: ['pullover']
true labels: ['pullover']