[image pre-processing] image normalization

def image_normalization(image_batch, batch_size):

    for i in range(0, batch_size):
        # get i-th image from the input mini-batch
        tmp = image_batch[i, :, :, :]

        # calculate std and mean of the image
        std = np.sqrt(np.var(tmp.astype("float32"))) + 0.00000001
        mean = np.mean(tmp.astype("float32"))

        # normalization
        image_batch[i, :, :, :] = (tmp.astype("float32") - mean) / std

    return image_batch