tf.Graph.as_default()
as_default(self)
Returns a context manager that makes this `Graph` the default graph.
This method should be used if you want to create multiple graphs in the same process. For convenience, a global default graph is
provided, and all ops will be added to this graph if you do not create a new graph explicitly. Use this method with the `with` keyword
to specify that ops created within the scope of a block should be added to this graph.
原文：http://www.cnblogs.com/max-hu/p/7072426.html