DYNAMIC_UNBOUND support for portable runtime: lazy KV cache allocation #11886

Job	Run time
test-metal-qwen35-moe-tiny / macos-job	5m 33s
test-metal-backend-modules / macos-job	6m 2s
export-model-metal-artifact (openai, whisper-large-v3-turbo, non-quantized) / macos-job	6m 28s
export-model-metal-artifact (nvidia, parakeet-tdt, quantized-int4-metal) / macos-job	6m 16s
export-model-metal-artifact (openai, whisper-large-v3-turbo, quantized-int4-metal) / macos-job	6m 29s
export-model-metal-artifact (openai, whisper-small, quantized-int4-metal) / macos-job	6m 21s
export-model-metal-artifact (mistralai, Voxtral-Mini-3B-2507, quantized-int4-metal) / macos-job	6m 32s
test-executorch-metal-build / macos-job	4m 51s
export-model-metal-artifact (mistralai, Voxtral-Mini-4B-Realtime-2602, quantized-int4-metal) / macos-job	12m 51s
export-model-metal-artifact (openai, whisper-small, non-quantized) / macos-job	6m 19s
export-model-metal-artifact (mistralai, Voxtral-Mini-3B-2507, non-quantized) / macos-job	6m 27s
export-model-metal-artifact (nvidia, parakeet-tdt, non-quantized) / macos-job	6m 18s
test-model-metal-e2e	-1s
	1h 20m 26s

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