Mxnet基础知识（一）

 1. 基本数据结构

　　和pytorch等中的tensor类似，mxnet中的ndarray或者nd，用来操作矩阵或者张量数据。基本操作类似于Numpy， 支持计算，索引等。

      创建矩阵

from mxnet import nd   #或者 from mxnet import ndarray as nd

#创建矩阵
x1 = nd.array([[1, 2,], [3, 4]])
x2 = nd.random.uniform(1, 10, shape=(3, 3))   #3*3的矩阵
x3 = nd.random.randn(2,3)  #2*3 的矩阵
x4 = nd.random.randint(1, 10, shape=(2, 3)) #2*3 的矩阵
x5 = nd.ones(shape=(2, 2))  #2*2 的矩阵
x6 = nd.full(shape=(2, 3), val=2)  #2*3 的矩阵, 值为2
print(x1.shape, x1.size, x1.dtype)  #(2, 2)   4   <class 'numpy.float32'>

　　操作矩阵

x = nd.random.randn(2, 3)
y = nd.random.randn(2, 3)
print(y.exp())  # 2*3 的矩阵
print(x*y)  # 2*3 的矩阵
print(nd.dot(x, y.T))  # 2*2 的矩阵

#和numpy相互转换
a = y.asnumpy()
print(a)
a = nd.array(np.ones((2, 3)))
print(a)

　　矩阵索引

y = nd.random.randint(1, 10, shape=(3, 3))
print(y[1, 2]) # [5]
print(y[:, 1:3]) # 3*2
y[:,1:3] = 2   #赋值
y[1:2,0:2] = 4  #赋值
print(y)

2. 创建神经网络

　　mxnet中gluon包中包含神经网络创建中的相关操作，和pytorch类似，可以继承block来创建神经网络，只需定义网络结构和实现前向传播函数。

　　方式一： 继承nn.Block

class MyNet(nn.Block):
    def __init__(self):
        super(MyNet, self).__init__()
        self.features = nn.Sequential()
        self.features.add(
            nn.Conv2D(channels=16, kernel_size=5, strides=(1, 1),
                      padding=(0, 0), activation="relu"),  #和pytorch不同之处：不需要设置输入通道数，可以设置激活函数
            nn.MaxPool2D(pool_size=(2, 2), stides=2, padding=0),
            nn.Conv2D(channels=32, kernel_size=3, strides=(1, 1),
                      padding=(0, 0), activation="relu"),
            nn.MaxPool2D(pool_size=(2, 2), stides=2, padding=0),

        )

        self.fc = nn.Sequential()
        self.fc.add(
            nn.Dense(units=120, activation="relu"),  #和pytorch不同之处：不需要设置输入向量的大小，可以设置激活函数
            nn.Dense(units=84, activation="relu"),
            nn.Dense(units=10)
        )

    def forward(self, x):
        x = self.features(x)
        x = self.fc(x)
        return x

net = MyNet()
net.initialize()   # 网络内部的参数必须先进行初始化 (pytorch中需要逐层进行初始化)
x = nd.random.uniform(shape=(1, 3, 300, 300))
print(net(x))

 　　方式二：直接利用nn.Sequential

net = nn.Sequential()
net.add(
    nn.Conv2D(channels=16, kernel_size=5, strides=(1, 1),
              padding=(0, 0), activation="relu"),  # 和pytorch不同之处：不需要设置输入通道数，可以设置激活函数
    nn.MaxPool2D(pool_size=(2, 2), strides=2, padding=0),
    nn.Conv2D(channels=32, kernel_size=3, strides=(1, 1),
              padding=(0, 0), activation="relu"),
    nn.MaxPool2D(pool_size=(2, 2), strides=2, padding=0),
    nn.Dense(units=120, activation="relu"),  # 和pytorch不同之处：不需要设置输入向量的大小，可以设置激活函数
    nn.Dense(units=84, activation="relu"),
    nn.Dense(units=10)
)

net.initialize()   # 网络内部的参数必须先进行初始化 (pytorch中需要逐层进行初始化)
x = nd.random.uniform(shape=(1, 3, 300, 300))
print(net(x))

3. 神经网络训练

　　梯度反向传播，mxnet会自动求导，需要利用mxnet的autograd,如下：

from mxnet import nd
from mxnet import autograd

x = nd.array([[1, 2], [3, 4]])
x.attach_grad()  #1. 声明存储导数的地方
with autograd.record():   #2. 该上下文中的过程，反向传播时会自动求导
    y = 2*x*x
y.backward()          #3. 反向传播; 会自动求和再计算导数，相当于y.sum().backward()
print(x.grad)          #4. 取导数值

 　　3.1. 加载数据

　　自己加载数据，主要需要继承mxnet.gluon.data.Dataset，然后传递给mxnet.gluon.data.DataLoader。有几个坑：

　　A.  Dataset返回img和label， label不能为字符串格式

　　B. Dataloader中的num_workers设置大于0时, 对于windows系统，由于采用多进程，需要写在__main__中；若还是报错时，num_workers改为0

#coding:utf-8
import mxnet as mx
from mxnet import gluon
from mxnet.gluon.data import Dataset, DataLoader
from mxnet.gluon.data.vision import transforms
import os
import cv2


#1.继承mxnet.gluon.data.Dataset, 实现__len__和__getitem__（返回每张图片和标注）
class MyDataset(Dataset):
    def __init__(self, img_root, anno_file):
        assert os.path.exists(anno_file), print("Annotation file {} not exist".format(anno_file))
        self.img_root = img_root
        self.anno_file = anno_file
        with open(anno_file, "r", encoding="utf-8") as f:
            lines = f.readlines()
        self.items = [line.strip().split() for line in lines if line.strip()]

    def __len__(self):
        return len(self.items)

    def __getitem__(self, x):
        img_name, label = self.items[x]
        img_path = os.path.join(self.img_root, img_name)
        assert os.path.exists(img_path), print("img_file {} does not exist".format(img_path))
        img = mx.image.imread(img_path)

        return img, label   #注意此处label为字符串会报错

if __name__ == "__main__":
    #2. 将dataset传入mxnet.gluon.data.Dataloader
    img_root = r"D:\data\synthtext"
    anno_file = r"D:\data\synthtext\labels.txt"
    dataset = MyDataset(img_root, anno_file)
    transformer = transforms.Compose([
        transforms.ToTensor(),
        transforms.Normalize(mean=(0.485, 0.456, 0.406), std=(0.229, 0.224, 0.225))
    ])  # dataset.transform_first(transformer), 对图片进行增强(即对__getitem__返回的第一项进行处理)
    train_data = DataLoader(dataset.transform_first(transformer), batch_size=2, shuffle=True, num_workers=0)
    print(train_data)
    for img, label in train_data:
        print(label)
        print(img.shape)

　　3.2 定义网络

　　见文章上面第二点

　　3.3 定义损失函数　　

　　gluon.loss包含了部分常用的Loss，如下：

 　　loss = gluon.loss.SoftmaxCrossEntropyLoss()   #交叉熵损失函数
    loss = gluon.loss.L2Loss()                  #均方差损失函数
    loss = gluon.loss.CTCLoss()                 # CTC损失函数
    loss = gluon.loss.L1Loss()                  # L1 损失函数
    #位找到smoothL1,发现两个相关的 mx.nd.smooth_l1()； mx.metric.Loss("SmoothL1")

_all__ = ['Loss', 'L2Loss', 'L1Loss',
           'SigmoidBinaryCrossEntropyLoss', 'SigmoidBCELoss',
           'SoftmaxCrossEntropyLoss', 'SoftmaxCELoss',
           'KLDivLoss', 'CTCLoss', 'HuberLoss', 'HingeLoss',
           'SquaredHingeLoss', 'LogisticLoss', 'TripletLoss', 'PoissonNLLLoss', 'CosineEmbeddingLoss']

　　3.4 定义优化器

 　　优化器定义在gluon.Trainer() ，第一个参数params为网络参数，第二个参数optimizer为优化器的名字，第三个参数optimizer_params为传给优化器的参数

　　 支持的optimizer如下：

 __all__ = [
        'AdaDelta', 'AdaGrad', 'Adam', 'Adamax', 'DCASGD', 'FTML', 'Ftrl', 'LBSGD',
        'NAG', 'NDabs', 'Nadam', 'Optimizer', 'RMSProp', 'SGD', 'SGLD', 'Signum',
        'Test', 'ccSGD', 
    ]

　　共同支持的optimizer_params如下： （不同优化器还有其特定的参数）

Parameters
rescale_grad (float, optional, default 1.0) – Multiply the gradient with rescale_grad before updating. Often choose to be 1.0/batch_size.

param_idx2name (dict from int to string, optional, default None) – A dictionary that maps int index to string name.

clip_gradient (float, optional, default None) – Clip the gradient by projecting onto the box [-clip_gradient, clip_gradient].

learning_rate (float) – The initial learning rate. If None, the optimization will use the learning rate from lr_scheduler. If not None, it will overwrite the learning rate in lr_scheduler. If None and lr_scheduler is also None, then it will be set to 0.01 by default.

lr_scheduler (LRScheduler, optional, default None) – The learning rate scheduler.

wd (float, optional, default 0.0) – The weight decay (or L2 regularization) coefficient. Modifies objective by adding a penalty for having large weights.

sym (Symbol, optional, default None) – The Symbol this optimizer is applying to.

begin_num_update (int, optional, default 0) – The initial number of updates.

multi_precision (bool, optional, default False) – Flag to control the internal precision of the optimizer. False: results in using the same precision as the weights (default), True: makes internal 32-bit copy of the weights and applies gradients in 32-bit precision even if actual weights used in the model have lower precision. Turning this on can improve convergence and accuracy when training with float16.

param_dict (dict of int -> gluon.Parameter, default None) – Dictionary of parameter index to gluon.Parameter, used to lookup parameter attributes such as lr_mult, wd_mult, etc. param_dict shall not be deep copied.

aggregate_num (int, optional, default None) – Number of weights to be aggregated in a list. They are passed to the optimizer for a single optimization step. In default, only one weight is aggregated. When aggregate_num is set to numpy.inf, all the weights are aggregated.

use_fused_step (bool, optional, default None) – Whether or not to use fused kernels for optimizer. When use_fused_step=False, step is called, otherwise, fused_step is called.

Properties –

---------- –

learning_rate – The current learning rate of the optimizer. Given an Optimizer object optimizer, its learning rate can be accessed as optimizer.learning_rate.

optimizer_params

　　常用优化器使用如下：

#优化器
    #1.动量法
    gluon.Trainer(params=net.collect_params(), optimizer="SGD",
                  optimizer_params={"learning_rate":0.001, "wd":0.00005, "momentum":0.9})
    #2. 自适应
    #AdaGrad
    gluon.Trainer(params=net.collect_params(), optimizer="AdaGrad",
                  optimizer_params={"learning_rate":0.001, "wd":0.00005,})
    #RMSProp
    gluon.Trainer(params=net.collect_params(), optimizer="RMSProp",
                  optimizer_params={"learning_rate": 0.001, "wd": 0.00005, "momentum":0.9})
    #Adam
    gluon.Trainer(params=net.collect_params(), optimizer="RMSProp",
                  optimizer_params={"learning_rate": 0.001, "wd": 0.00005})

　　3.5 模型训练

    for epoch in range(10):
        train_loss, train_acc, valid_acc = 0., 0., 0.
        tic = time.time()
        for data, label in train_data:
            # forward + backward
            with autograd.record():
                output = net(data)
                loss = softmax_cross_entropy(output, label)
            loss.backward()
            # update parameters
            trainer.step(batch_size)
            # calculate training metrics
            train_loss += loss.mean().asscalar()
            train_acc += acc(output, label)
        # calculate validation accuracy
        for data, label in valid_data:
            valid_acc += acc(net(data), label)
        print("Epoch %d: loss %.3f, train acc %.3f, test acc %.3f, in %.1f sec" % (
            epoch, train_loss / len(train_data), train_acc / len(train_data),
            valid_acc / len(valid_data), time.time() - tic))

4. 网络参数保存和加载　　

　　Block 只能保存网络参数，如下：

　　net = nn.Sequential()
　　net.add(
    nn.Conv2D(channels=16, kernel_size=5, strides=(1, 1),
              padding=(0, 0), activation="relu"),  # 和pytorch不同之处：不需要设置输入通道数，可以设置激活函数
    nn.MaxPool2D(pool_size=(2, 2), strides=2, padding=0),
    nn.Conv2D(channels=32, kernel_size=3, strides=(1, 1),
              padding=(0, 0), activation="relu"),
    nn.MaxPool2D(pool_size=(2, 2), strides=2, padding=0),
    nn.Dense(units=120, activation="relu"),  # 和pytorch不同之处：不需要设置输入向量的大小，可以设置激活函数
    nn.Dense(units=84, activation="relu"),
    nn.Dense(units=10)
)

　　#1 保存网络权重参数
    net.save_parameters("checkpoint.params")
　　#2 加载权重参数
    net.load_parameters("checkpoint.params", ctx=None, allow_missing=False,
                        ignore_extra=False, cast_dtype=False, dtype_source='current')
        ctx: 默认为Cpu
        allow_missing: True时表示：网络结构中存在, 参数文件中不存在参数，不加载
        ignore_extra:   True时表示: 参数文件中存在，网络结构中不存在的参数，不加载

　　

　　HybridBlock可以向Block一样保存网络参数，也可以同时保存网络结构和网络参数， 如下：

    net = nn.HybridSequential()
    net.add(
        nn.Conv2D(channels=16, kernel_size=5, strides=(1, 1),
                  padding=(0, 0), activation="relu"),  # 和pytorch不同之处：不需要设置输入通道数，可以设置激活函数
        nn.MaxPool2D(pool_size=(2, 2), strides=2, padding=0),
        nn.Conv2D(channels=32, kernel_size=3, strides=(1, 1),
                  padding=(0, 0), activation="relu"),
        nn.MaxPool2D(pool_size=(2, 2), strides=2, padding=0),
        nn.Dense(units=120, activation="relu"),  # 和pytorch不同之处：不需要设置输入向量的大小，可以设置激活函数
        nn.Dense(units=84, activation="relu"),
        nn.Dense(units=10)
    )
    # 1.对于HybridBlock, 可以同时保存网络结构和权重参数
    #首先要进行hybridize()和一次前向传播，才能进行export
    net.initialize()
    net.hybridize()
    x = mx.nd.zeros((1, 3, 100, 100))
    print(net(x))
    net.export(path="./checkpoint", epoch=1) #同时生成checkpoint-0001.params 和 checkpoint-symbol.json
    # # net.save_parameters("./checkpoint.params")

　　#2. 加载export的网络结构(json)和权重参数(params)
　　#或者mx.SymbolBlock.imports()
　　net = gluon.SymbolBlock.imports(symbol_file="./checkpoint-symbol.json",
                          input_names=["data"],
                          param_file="./checkpoint-0100.params",
                          ctx=mx.cpu())
　　net.hybridize()
　　x = mx.nd.zeros((1, 3, 100, 100))
　　print(net(x))
　　# net = mx.mod.Module.load(prefix="./checkpoint", epoch=100)

5. 使用GPU

　　在进行训练和计算时网络参数和数据必须在同一环境下，同在CPU或同在GPU，采用GPU计算矩阵时能加速运算；可以在GPU上操作数据和网络，如下：

　　数据：可以在GPU上创建数据，也可以在CPU上创建数据，载移动到GPU

#1. 在GPU上创建数据，或者将数据从cpu移动到GPU
    #GPU创建
    x = mx.nd.zeros((1, 3, 100, 100), ctx=mx.gpu(0))
    print(x)
    #cpu创建，复制一份到GPU
    x = mx.nd.zeros((1, 3, 100, 100))
    x = x.copyto(mx.gpu(0))
    print(x)
    # cpu创建，复制一份移动到GPU
    x = mx.nd.zeros((1, 3, 100, 100))
    x = x.as_in_context(mx.gpu(0))
    print(x)

　　网络：可以在GPU上加载网络参数，或者在CPU上加载，随后移动到GPU

#2.在GPU上加载网络参数，或者将网络参数移动到GPU
    net = nn.Sequential()
    net.add(
        nn.Conv2D(channels=16, kernel_size=3, strides=1, padding=1),
        nn.Dense(18)
    )
    #GPU上初始化参数
    net.initialize(init=mx.init.Xavier(), ctx=mx.gpu(0))
    net.load_parameters("./checkpoint.params", ctx=mx.gpu(0))

    # #CPU上初始化参数，移动到GPU
    net.initialize(init=mx.init.Xavier())
    net.collect_params().reset_ctx(mx.gpu())

https://github.com/apache/incubator-mxnet

https://zhuanlan.zhihu.com/p/39420301

http://mxnet.incubator.apache.org/