perf(coreml): run decode on the CPU, not the Neural Engine

decode is a single-token program whose cost on device is dispatch rather than arithmetic, so it was paying Neural Engine overhead on every call under ExecuTorch's ALL default. Pinning only decode to CPU_ONLY (encode stays on ALL) measures 6.7 ms/step against 20.0 on an iPhone 16.

On 14.5s of real speech the whole transcribe is 1.5-3.1x faster depending on size, with a byte-identical transcript: this removes overhead, it does not change numerics.

Exported from export-scripts MR !18.

coreml/whisper_small_coreml_fp16.pte

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