Move where cuda implementation to TensorIterator (#32984)
Summary:
`where` is special because the arguments do not have the same type, which does not satisfy the assumption in modern https://github.com/pytorch/pytorch/pull/32383. I migrate it to TensorIterator so that there is something to test that this case is not broken. Currently, this case fallback to using legacy (not vectorized, not unrolled) code. It should be supported in the future when I cleanup `Loops.cuh`.
I also move some sharing part of `CUDALoops.cuh` and `ROCmLoops.cuh` into `Loops.cuh` so that to logic for checking whether `func_t` has the same arg types could be shared.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32984
Differential Revision: D19825127
Pulled By: ngimel
fbshipit-source-id: bbf4682349d96b4480c4d657f3c18a3a67a9bf17
