Parallel: Deduplicate parallel functions in different backends (#65326)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/65326
parallel_for and parallel_reduce currently share some common code in
all backends, specifically for detecting if it should run in parallel
or not. This moves all the backend-specific code into a single
`internal::invoke_parallel` function and makes the `parallel_`
functions common to all backends.
Test Plan: Imported from OSS
Reviewed By: ezyang
Differential Revision: D31124495
fbshipit-source-id: 65c3d2af42a8860cc4d6349566085c9fa8d8c6f0
