fix upsampling decompositions to have integer output sizes (#94123)
This allows unet to be compiled with symbolic shapes (but it still fails accuracy, lol).
Output sizes are always integer, there's no need to pretend they are ever float. Recomputing scale factors still used nominally float sizes converted to int, we might as well do it from the start.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/94123
Approved by: https://github.com/ezyang
