Add optimized Arm64 BF16 _to_copy

[JakeStevens]

Summary: Add an optimized _to_copy.out kernel. On Arm64, contiguous FP32-to-BF16 and BF16-to-FP32 conversions process eight elements per NEON iteration and use the ExecuTorch threadpool above its grain size. Other dtype pairs, layouts, and platforms fall back to the portable implementation, which remains unchanged.

Differential Revision: D118502073

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/22493

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❌ 5 New Failures

As of commit 4ee4411 with merge base 4747ab7 ():

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• Cadence Build & Test / vision-build / vision (gh)
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• pull / unittest / linux / linux-job (gh)
  pytorch_tokenizers
• pull / unittest-buck / linux / linux-job (gh)
  pytorch_tokenizers
• pull / unittest-editable / linux / linux-job (gh)
  pytorch_tokenizers

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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 D118502073.

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