[core] Get rid of signal handling

[Michaelvll]

We currently use signal handling in our backend to kill the child processes immediately after keyboard interrupt is sent and print the hints. However, this will cause sky.launch not able to be run in a non-main thread. This is a bug encountered by a user, and we should look into it.

skypilot/sky/backends/cloud_vm_ray_backend.py

Lines 3257 to 3258 in ac9ba6a

	signal.signal(signal.SIGINT, backend_utils.interrupt_handler)
	signal.signal(signal.SIGTSTP, backend_utils.stop_handler)

[concretevitamin]

Just ran into this too when trying out #2235.

[concretevitamin]

A user reported they sometimes ran into this during sky.launch(), with the two detach flags set to True.