tf.image.convert_image_dtype 注意

image: 3-D Tensor of image. If dtype is tf.float32 then the range should be
[0, 1], otherwise it would converted to tf.float32 assuming that the range
is [0, MAX], where MAX is largest positive representable number for
int(8/16/32) data type (see `tf.image.convert_image_dtype` for details).

如果是float32，那麼需要自己保證輸入值在0-1之間，否則是按照除以輸入類型最大值歸一化的。