https://github.com/tensorflow/tensorflow/blob/2de01b1a7334522314b410d53aeeb0262057369a/tensorflow/contrib/slim/python/slim/nets/alexnet.py#L41中的第一次导入和第二次导入的图层之间有什么区别
tensorflow.contrib import layers
和
from tensorflow.contrib.layers.python.layers import layers as layers_lib
类似地，
from tensorflow.python.ops import variable_scope
和
from tensorflow.contrib.framework.python.ops import variables as variables_lib