VersionDef

 Version information for a piece of serialized data
 There are different types of versions for each type of data
 (GraphDef, etc.), but they all have the same common shape
 described here.
 Each consumer has "consumer" and "min_producer" versions (specified
 elsewhere).  A consumer is allowed to consume this data if
   producer >= min_producer
   consumer >= min_consumer
   consumer not in bad_consumers