Indicates when a distributed variable will be synced.
AUTO: Indicates that the synchronization will be determined by the current
MirroredStrategythis would be
NONE: Indicates that there will only be one copy of the variable, so there is no need to sync.
ON_WRITE: Indicates that the variable will be updated across devices every time it is written.
ON_READ: Indicates that the variable will be aggregated across devices when it is read (eg. when checkpointing or when evaluating an op that uses the variable).