The feedback is in the operation of the differential pair itself. By feeding it with a current source itail, common signals don't cause changes in the drain currents.
i figure d it out.
@fvm:
the i-tail is introduced later . initially it was grounded, even with ground there is an inherent common mode feed forward in the circuit. what we are doing
is , extracting the common mode signal from the output. here the average current is prevented from going in to the load. so no common mode voltage is developed.