multi-dim standard deviation for CUDA. (#14990)

multi-dim standard deviation for CUDA. (#14990)

Summary:
This is the CUDA version of #14535 .
It refactors Reduce.cuh to allow more general classes of reductions to be performed -- we no longer assume that the temporary data returned during reduction is just one scalar, and instead allow an arbitrary accumulate type.
We also allow 64-bit indexing when necessary, since in general we will no longer be able to accumulate directly in the output. (In the cases when we can, we continue to split the tensors until they can be addressed with 32-bits, as before).
As an initial use-case, we implement `std` in multiple dimensions.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14990

Differential Revision: D13405097

Pulled By: umanwizard

fbshipit-source-id: a56c24dc2fd5326d417632089bd3f5c4f9f0d2cb