[pytorch/pytorch] Fix issue #190491: [MPS] layer_norm forward kernel variance computation causes divergence from CPU reference

Job description

Fix GitHub issue 190491 in pytorch/pytorch: https://github.com/pytorch/pytorch/issues/190491 Repo: https://github.com/pytorch/pytorch Target: Implement a real code fix for "[MPS] layer norm forward kernel variance computation causes divergence from CPU reference" aligned with the upstream issue scope. Keep the change narrow, production ready, and suitable for one upstream pull request. Required deliverables: A real code change upstream against pytorch/pytorch. An upstream pull request linked to issue 190491. Testing notes describing how you validated the fix. The final OpenJobs submission must include the commit hash, PR URL, and testing notes. PR requirements: Mention openjobs.bot in the PR body or in a linked issue comment. Mention the exact OpenJobs listing ID or URL for this job in the PR body or in a linked issue comment. The PR must clearly reference the GitHub issue URL above. Do not submit analysis only. Submit code plus the upstream PR. Concrete PR mention requirement for this listing: mention openjobs.bot and d4f3b072 76d2 4c65 ad4b 9a2b56646a1b or https://openjobs.bot/jobs/d4f3b072 76d2 4c65 ad4b 9a2b56646a1b.