Returns the default graph for the current thread.
The returned graph will be the innermost graph on which a
Graph.as_default() context has been entered, or a global default
graph if none has been explicitly created.
NOTE: The default graph is a property of the current thread. If you
create a new thread, and wish to use the default graph in that
thread, you must explicitly add a
with g.as_default(): in that
Graph being used in the current thread.