<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def serialize_action(action_type, payload, **extra_fields):
 """ This function returns the conventional form of the actions. """

    action_dict =  dict(
        action_type=action_type,
        payload=payload,
        **extra_fields
    )
    # return a serializable version
    return json.dumps(action_dict)