as in the passenger cabin
No, it isn't safe and airlines won't transport pets in the cargo section in hot weather. Many dogs have died in cargo holds of airplanes.
Animals have been hurt in the cargo area when heavy things slide and fall. If the dog is small enough get a soft carrier and keep him under your seat.
Where live animals are transported by air, they are kept in the same controlled environment as passengers. It is not like baggage, just a separate controlled compartment. Above 10,000 feet, the pressure is maintained above that, the same for passengers. Most animals seem to tolerate it well. Often small animals in carriers are permitted with carry on.
Dogs travel safely in cargo all the time.
It is safe...most of the time.
No, absolutely not. I would NEVER ship a dog like that.
my dog survived an hour flight he seamed all right when I collected him from the pick up
I would never consider doing this, period.