Fix recognize digits by adding images

source/quick_start/recognize_digits/image/cnn.png

-../../../../book/02.recognize_digits/image/cnn.png
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source/quick_start/recognize_digits/image/cnn_train_log.png

-../../../../book/02.recognize_digits/image/cnn_train_log.png
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source/quick_start/recognize_digits/image/conv_layer.png

-../../../../book/02.recognize_digits/image/conv_layer.png
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source/quick_start/recognize_digits/image/infer_3.png

-../../../../book/02.recognize_digits/image/infer_3.png
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source/quick_start/recognize_digits/image/max_pooling.png

-../../../../book/02.recognize_digits/image/max_pooling.png
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source/quick_start/recognize_digits/image/mlp.png

-../../../../book/02.recognize_digits/image/mlp.png
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source/quick_start/recognize_digits/image/mlp_train_log.png

-../../../../book/02.recognize_digits/image/mlp_train_log.png
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source/quick_start/recognize_digits/image/mnist_example_image.png

-../../../../book/02.recognize_digits/image/mnist_example_image.png
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source/quick_start/recognize_digits/image/softmax_regression.png

-../../../../book/02.recognize_digits/image/softmax_regression.png
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source/quick_start/recognize_digits/image/softmax_train_log.png

-../../../../book/02.recognize_digits/image/softmax_train_log.png
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source/quick_start/recognize_digits/image/train_and_test.png

-../../../../book/02.recognize_digits/image/train_and_test.png
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