int32 indexing for Tensor Iterator Reduction (#17428)

int32 indexing for Tensor Iterator Reduction (#17428)

Summary:
1. Enabling int32 indexing for cases where TI cannot accumulate in output due to
incompatible data types (e.g. Welford).
2. Updating Welford kernel to use int32 instead of int64 indexing on GPU.

This change improves performance for torch.var / torch.std

Implementation:
1. Allocated extra buffer to handle accumulation between sub Tensor Iterators.
2. Removed int64 indexing in gpu_reduce_kernel
3. WelfordOps now supports index type / combination typeas a template parameter.
While GPU uses int32_t and float, CPU implementation uses int64_t and double.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17428

Differential Revision: D14264608

Pulled By: umanwizard

fbshipit-source-id: 3eb54451de925b469dbc1127e5ea7443c4431036