how do I smooth out the bottom of a hole in granite
So, how rough is the bottom of the hole?
Was the hole drilled in steps from small to large or was it a knocked out core.
Some ideas could be rotating wood dowel of a suitable diameter with sand to smooth out the surface.
How smooth does it need to be - as long as the tea light sits reasonably square then it should be suitable.
There are low speed grinders designed for finishing holes like that but they can usually be found at stonemasons or marble suppliers - you may consider asking there, but it depends on how big are your pieces and how many for the cost.