The cargo hold of a modern jet aircraft is pressurised and air conditioned just like the passenger cabin to minimise internal stress. Air is sucked in by the APU (on the ground) or engines (during flight), bled off and cooled by the air conditioning units (PAKs) and pumped into the fuselage. A release valve usually located at the rear of the plane allows air to vent, thus fresh air is continuously pumped in and cooled, while old air escapes via the relief valve, creating a circulating atmosphere of breathable air conditioned air inside the aircraft.
Your chocolates will generally be fine while inside the cabin or cargo hold. Of course, you should take into consideration that while your luggage is on the tarmac or being transferred it may be subject to the elements and outside air temperature. A beg left sitting on a trolly in the sun on a hot day can warm up very quickly.
On a final note, some aircraft designs have the PAKs (air conditioning units) located next to or around the cargo holds. On a very hot day, if the aircraft is sitting around for a while, the PAKs themselves can get quite hot, and that heat can transfer into nearby areas such as the hold where your chocolates are sitting. However, there is no equivalent to a car's radiator in a passenger jet.
The safest bet is to take the chocolates (or anything that can melt) as on board luggage. Just don't eat them on the way!