如何在ModelChoiceFieldoptgroup标签中设置?这是一个例子:models.pyclassLink(models.Model):config=models.ForeignKey(Config)name=models.URLField(u'Name',null=True,max_length=50)gateway=models.IPAddressField(u'Gateway',null=True)weight=models.IntegerField(u'Weight',null=True)description=models.TextField(u'Descripti