Auto ML 一种自动完成机器学习任务的系统

Auto ML 是能够自动化完成一些机器学习任务的系统,

在 2018 年比较火,很多大公司都开源了各自的auto ml库,例如 Cloud AutoML, AUTO KERAS, Auto Sklearn, Auto Weka 等,

并被很多数据科学家预测在 2019 年仍然是机器学习的热点。



在做一个机器学习项目时,几乎每个环节都要人为地进行各种处理,各种尝试

例如数据预处理环节,一般就需要做这些步骤:

text vectorization

categorical data encoding (e.g., one hot)

missing values and outliers processing

rescaling (e.g., normalization, standardization, min-max scaling)

variables discretization

dimensionality reduction


还需要选择算法:

supervised or not, classification or regression, online or batch learning


特征工程,参数调节也是更复杂的部分,而且没有一个标准的模式可以遵循,随问题而变化


Auto ML 的目的就是要减少人为的操作,将特征工程,模型参数设置,算法选择部分由这个系统自动地去完成,并且要达到更好的性能,更快地运算


主要的算法有:

用于自动寻找最优神经网络结构的 NAS算法,

用于搜索超参的 贝叶斯算法,TPE模型等,

还有Google的 Bandit 算法,以及比较经典的遗传算法




以 Keras 为例:

在深度学习的库中,Keras 已经算是很简单明了的了,建立一个神经网络结构也比较方便,下面我们看看用 Keras 做 MNIST 任务的代码:


from __future__ import print_function

import keras

from keras.datasets import mnist

from keras.models import Sequential

from keras.layers import Dense, Dropout, Flatten

from keras.layers import Conv2D, MaxPooling2D

from keras import backend as K


batch_size = 128

num_classes = 10

epochs = 12


# input image dimensions

img_rows, img_cols = 28, 28


# the data, split between train and test sets

(x_train, y_train), (x_test, y_test) = mnist.load_data()


if K.image_data_format() == 'channels_first':

    x_train = x_train.reshape(x_train.shape[0], 1, img_rows, img_cols)

    x_test = x_test.reshape(x_test.shape[0], 1, img_rows, img_cols)

    input_shape = (1, img_rows, img_cols)

else:

    x_train = x_train.reshape(x_train.shape[0], img_rows, img_cols, 1)

    x_test = x_test.reshape(x_test.shape[0], img_rows, img_cols, 1)

    input_shape = (img_rows, img_cols, 1)


x_train = x_train.astype('float32')

x_test = x_test.astype('float32')

x_train /= 255

x_test /= 255

print('x_train shape:', x_train.shape)

print(x_train.shape[0], 'train samples')

print(x_test.shape[0], 'test samples')


# convert class vectors to binary class matrices

y_train = keras.utils.to_categorical(y_train, num_classes)

y_test = keras.utils.to_categorical(y_test, num_classes)


model = Sequential()

model.add(Conv2D(32, kernel_size=(3, 3),

                activation='relu',

                input_shape=input_shape))

model.add(Conv2D(64, (3, 3), activation='relu'))

model.add(MaxPooling2D(pool_size=(2, 2)))

model.add(Dropout(0.25))

model.add(Flatten())

model.add(Dense(128, activation='relu'))

model.add(Dropout(0.5))

model.add(Dense(num_classes, activation='softmax'))


model.compile(loss=keras.losses.categorical_crossentropy,

              optimizer=keras.optimizers.Adadelta(),

              metrics=['accuracy'])


model.fit(x_train, y_train,

          batch_size=batch_size,

          epochs=epochs,

          verbose=1,

          validation_data=(x_test, y_test))

score = model.evaluate(x_test, y_test, verbose=0)

print('Test loss:', score[0])

print('Test accuracy:', score[1])



上面的代码中包含了下面这些步骤:

数据预处理,

设置模型参数,

建立模型,

训练模型,

评估模型



如果用 Auto-Keras 来做呢:


from keras.datasets import mnist

from autokeras.classifier import ImageClassifier


if __name__ == '__main__':

(x_train, y_train), (x_test, y_test) = mnist.load_data()

x_train = x_train.reshape(x_train.shape + (1,))

x_test = x_test.reshape(x_test.shape + (1,))


clf = ImageClassifier(verbose=True, augment=False)

clf.fit(x_train, y_train, time_limit=12 * 60 * 60)

clf.final_fit(x_train, y_train, x_test, y_test, retrain=True)

y = clf.evaluate(x_test, y_test)

print(y * 100)


只需要 2 行,就自动化了前面的 数据预处理,设置模型参数



学习资源:

https://towardsdatascience.com/auto-keras-or-how-you-can-create-a-deep-learning-model-in-4-lines-of-code-b2ba448ccf5e

你可能感兴趣的:(Auto ML 一种自动完成机器学习任务的系统)