If you want to create Nested view like Leaves Summary, by setting default group_by in Menu Action.

If you can refer to source code of Menu Action of Leaves Summary:

        <record model="ir.actions.act_window" id="open_company_allocation">
            <field name="name">Leaves Summary</field>
            <field name="res_model">hr.holidays</field>
            <field name="view_type">form</field>
            <field name="view_mode">tree,form</field>
            <field name="view_id" eval="view_holiday_simple"/>
            <field name="context">{'search_default_group_type': 1}</field>
            <field name="domain">[('holiday_type','=','employee'), ('state', '!=', 'refuse')]</field>
            <field name="search_view_id" ref="view_hr_holidays_filter"/>
        </record>

In this,  in context we have passed default group_by in context:

  <field name="context">{'search_default_group_type': 1}</field>

to give default group_by condition in context we have to first define that group_by condition in search view of model.

you can observe that, In search_view_id reference of search_view is given, in that view definition, we have set group_by condition:

 <filter name="group_type" string="Type" context="{'group_by':'holiday_status_id'}"/>

Likewise, you can do it for your module.

Hope, this helps..