I make some changes in the read_group function which is located in the odoo/server/openerp/models.
What I need now is to override this
Please find the example as below
from openerp import models
def read_group():
#YOUR OVERRIDDEN Function
models.BaseModel.read_group = read_group
For New API and old API compatibility issue you can also use _register_hook
as given in example on this link.
https://stackoverflow.com/a/34854721/1451220