Vectorize min + max and add fused minmax

[JakeStevens]

Summary:
Replace the iterator-returning std::min_element / std::max_element implementations of torch::executor::vec_minf and vec_maxf with four independent value-reduction lanes. A shared compile-time implementation computes only the requested extrema, with a narrowly scoped Clang vectorization hint for NEON/SSE2 targets and no fast-math requirement.

Add torch::executor::vec_minmaxf(const float* x, size_t size, float* min_out, float* max_out) to compute both extrema together. Wire the per-tensor and both serial/parallel per-token choose_qparams paths to the fused helper, eliminating their separate minimum and maximum scans without changing scale/zero-point calculations.

Differential Revision: D119391738

[pytorch-bot]

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❌ 5 New Failures, 1 Cancelled Job, 2 Unrelated Failures

As of commit 7012c19 with merge base f03eeef ():

NEW FAILURES - The following jobs have failed:

• Cadence Build & Test / cpu-test / Resolve CI docker image / resolve (gh)
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• Cadence Build & Test / hifi-build / hifi4 (gh)
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• Cadence Build & Test / vision-build / vision (gh)
  ##[error]Refusing to check out fork pull request code from a 'pull_request_target' workflow. This workflow runs with the base repository's GITHUB_TOKEN, secrets, default-branch cache scope, and runner access. Fetching and executing a fork's code in that trusted context commonly leads to "pwn request" vulnerabilities. To opt in, review the risks at https://gh.io/securely-using-pull_request_target and set 'allow-unsafe-pr-checkout: true' on the actions/checkout step.
• Lint / lintrunner (gh)
  >>> Lint for kernels/portable/cpu/vec_ops.h:
• pull / test-binary-size-linux-gcc / linux-job (gh)
  /__w/executorch/executorch/pytorch/executorch/../executorch/kernels/portable/cpu/vec_ops.h:45:11: error: variable ‘mins’ set but not used [-Werror=unused-but-set-variable]

CANCELLED JOB - The following job was cancelled. Please retry:

• pull / unittest-editable / macos / macos-job (gh)
  ##[error]The operation was canceled.

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Test QNN Windows MSVC build / build-qnn-windows-arm64 (gh) (trunk failure)
• Test QNN Windows MSVC build / build-qnn-windows-x64 / windows-job (gh) (trunk failure)

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[meta-codesync]

@JakeStevens has exported this pull request. If you are a Meta employee, you can view the originating Diff in D119391738.

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