Add code to get mean, standard deviation for mnist

mean_std.py

@@ -0,0 +1,23 @@
+# https://stackoverflow.com/a/53570674
+import gzip
+import numpy as np
+
+f = gzip.open('train-images-idx3-ubyte.gz','r')
+
+image_size = 28
+num_images = 60000
+
+f.read(16)
+buf = f.read(image_size * image_size * num_images)
+data = np.frombuffer(buf, dtype=np.uint8).astype(np.float32)
+data = data.reshape(num_images, image_size, image_size, 1)
+
+data = data.reshape((60000, 784))
+data /= 255
+data_mean = data.mean(0)
+data_mean = data.mean()
+
+data_std = data.std(0)
+data_std = data.std()
+
+print(data_mean, data_std)      # 0.13066062 0.30810776