Wrap partition tags as part of partition result (#269)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/269

`partition_tags` is an output from the `partition` function and it'll be cleaner to wrap it as part of the outputs instead of the instance attributes. This diff is supposed be a noop

Reviewed By: mergennachin, angelayi

Differential Revision: D49116131

fbshipit-source-id: 0943c6367e2aa902e28af7f058bd2904f871beac