Global variables are variables that are shared across machines in a
distributed environment. The Variable() constructor or get_variable()
automatically adds new variables to the graph collection
This convenience function returns the contents of that collection.
(Optional.) A string. If supplied, the resulting list is filtered to
include only items whose name attribute matches scope using
re.match. Items without a name attribute are never returned if a scope
is supplied. The choice of re.match means that a scope without special
tokens filters by prefix.