tf.compat.v1.GraphDef
https://www.tensorflow.org/api_docs/python/tf/compat/v1/GraphDef
GraphDef有很多NodeDef
https://www.tensorflow.org/api_docs/python/tf/compat/v1/NodeDef
class 'tensorflow.python.framework.ops.Graph'
class 'tensorflow.core.framework.graph_pb2.GraphDef //protobuf message.用以序列化成二进制文件及反序列化二进制文件后存储数据
graph.as_graph_def()完成从Graph到GraphDef的转换.