Fix portable kernel utils for aten mode (#1563)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1563

In supporting build for aten mode, reduce_util has dependency on
index_util.cpp. Reduce util depends on index_util for a util function,
check_index_out_args
However this function is used only when not being built for aten mode.
When being built with aten mode, index_util.cpp ends up including
c10/ScalarType.h (via scalar_type_utils.h). Both c10/ScalartType.h and
scalar_type_utils.h define toString which results in compiler error.

We should really fix scalar_type_utils.h to allow for aten mode build,
but that needs more work.

In the mean time check_index_out_args is used only in reduce_util.cpp so there is no
need to really factor out index_util.cpp, so removin this refactor.
ghstack-source-id: 211679772
exported-using-ghexport

Reviewed By: larryliu0820, GregoryComer

Differential Revision: D52626505

fbshipit-source-id: 57d4fdda77117347813738d86d611f8c17d5e660