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