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_tiny_coreml_fp16.pte

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:e140cf064b6a1aab4a857cdd7cb5684a1234a8fc289782e8184fe523d09db161
-size 76477038
+oid sha256:8a743e18747f925b49acea2a2be62c10a037cc2d6d63e3cb3740f08cdf9b70f5
+size 76478434