Consolidate MeanToSumDiv export pass into VulkanPreprocess (#4178)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4178

Clean up time. Previous issue fixed by registering `aten.mean.dim` in `supported_ops.py`.
ghstack-source-id: 233085714

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Reviewed By: copyrightly

Differential Revision: D59525641

fbshipit-source-id: 92be430ccdf30c78314110686709a83c435465fe