Graph.get_collection_ref() using the default graph.
for more details.
key: The key for the collection. For example, the
GraphKeysclass contains many standard names for collections.
The list of values in the collection with the given
name, or an empty
list if no value has been added to that collection. Note that this returns
the collection list itself, which can be modified in place to change the
Collections are not supported when eager execution is enabled.