Code details

Thanks for your elegant work !

I have a tiny question. it seems that you choose to filter different heads of attn_maps via a [pre-difined index order](https://github.com/linsun449/iseg.code/blob/2e8fb3210052c884e227e60e2d8e2824679bfa24/wsss/iSeg.py#L30). 

Is it a carefully-tuned trick? or could you kindly explain why the order is specifically choosed ?