mxnet/gluon學習系列（一）訓練結構概覽

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龜龜是最可愛的小貓咪

Gluon訓練一個網絡的結構概覽

從下面一段代碼中可以看到gluon定義及訓練一個網絡的基本過程

from mxnet import nd
from mxnet.gluon import nn
from mxnet import gluon
from mxnet import autograd

class Net(nn.Block):
    def __init__(self, **kwargs):
        super(Net, self).__init__(**kwargs)
        self.dense0 = nn.Dense(4, use_bias=False)
        self.dense1 = nn.Dense(2, use_bias=False)

    def forward(self, x):
        return self.dense1((self.dense0(x)))


def train():
    net = Net()
    net.initialize()
    w = net.dense0.weight
    print ('weight shape after initialize', w.shape)
    # 'weight params', w.data())
    trainer = gluon.Trainer(net.collect_params(), 'sgd', {'learning_rate': 1})
    data = nd.ones(shape=(1, 1, 28, 28))
    label = nd.ones(shape=(1, 2))
    loss = gluon.loss.L2Loss()
    with autograd.record():
        res = net(data)
        w = net.dense0.weight
        print ('net[0] name', net.dense0.name, 'weight shape', w.shape, '\nparams', w.data(), 'grad', w.grad())
        L = loss(res, label)
    L.backward()
    trainer.step(batch_size=1)
    print ('net[0] name', net.dense0.name, 'weight shape', w.shape, '\nparams', w.data(), 'grad', w.grad())


if __name__ == '__main__':
    train()

打印結果如下：

weight shape after initialize (4, 0)
net[0] name dense0 weight shape (4, 784)
params
[[ 0.0068339   0.01299825  0.0301265  ... -0.02863884 -0.065321
  -0.05844658]
 [ 0.06397291  0.03163359 -0.0507907  ...  0.03343763 -0.03537887
  -0.03153095]
 [ 0.02405292 -0.00215812  0.00864208 ... -0.03622346 -0.00675231
   0.01282176]
 [ 0.01297587  0.02718969 -0.02421831 ... -0.05000407  0.02510548
   0.00611195]]
<NDArray 4x784 @cpu(0)> grad
[[0. 0. 0. ... 0. 0. 0.]
 [0. 0. 0. ... 0. 0. 0.]
 [0. 0. 0. ... 0. 0. 0.]
 [0. 0. 0. ... 0. 0. 0.]]
<NDArray 4x784 @cpu(0)>
net[0] name dense0 weight shape (4, 784)
params
[[-0.02588698 -0.01972263 -0.00259437 ... -0.06135972 -0.09804188
  -0.09116746]
 [ 0.03434887  0.00200954 -0.08041474 ...  0.00381359 -0.06500292
  -0.06115499]
 [ 0.03586442  0.00965339  0.02045358 ... -0.02441196  0.00505919
   0.02463327]
 [-0.03283395 -0.01862013 -0.07002813 ... -0.09581389 -0.02070434
  -0.03969787]]
<NDArray 4x784 @cpu(0)> grad
[[ 0.03272087  0.03272087  0.03272087 ...  0.03272087  0.03272087
   0.03272087]
 [ 0.02962404  0.02962404  0.02962404 ...  0.02962404  0.02962404
   0.02962404]
 [-0.01181151 -0.01181151 -0.01181151 ... -0.01181151 -0.01181151
  -0.01181151]
 [ 0.04580982  0.04580982  0.04580982 ...  0.04580982  0.04580982
   0.04580982]]
<NDArray 4x784 @cpu(0)>

Process finished with exit code 0

主要步驟可以寫成：

1. 定義一個神經網絡
2. 網絡初始化
3. 訓練：
   • 定義一個Trainer
   • 傳入數據及標籤，數據大小batch_shape
   • 前向傳播計算loss
   • 反向傳播得到梯度
   • 更新權重等參數

通過一個示例來看訓練過程