Thanks Archmage, I tried both of your suggestions,

1. The first suggestion works there is only one baseboard on the pump loop. When i keep the pump flow rate fixed, It caps the flow rate on baseboard to that value but it is required still fluctuates between 0 and that value. Even if i try to keep the thermostat to a value pretty high (like 50 c), that results in severe errors with crazy high temperature and remove the bypass pipe.points.

2. I put a loadprofile:plant after the baseboard with zero load but controlling that object flow rate can not overwrite the base board flow rate.