Send the "group_by" attribute inside context tag of your action.
Ex:- {'group_by':['partner_id']}

Thanks for your answer. I try to add {'group_by':['partner_id']} in context on the window action, the result is real group by but NOT expand in each group. I would like to expand group like this picture https://www.dropbox.com/s/u54wxm1s8vwn8cf/Selection_353.png. 

Do you have any idea for do this ?