Get instances of all well known flows.

def GetAllWellKnownFlows(cls, token=None):
    """Get instances of all well known flows."""
    well_known_flows = {}
    for cls in itervalues(registry.AFF4FlowRegistry.FLOW_REGISTRY):
      if issubclass(cls, WellKnownFlow) and cls.well_known_session_id:
        well_known_flow = cls(cls.well_known_session_id, mode="rw", token=token)
        well_known_flows[cls.well_known_session_id.FlowName()] = well_known_flow

    return well_known_flows