Add workflow for on-demand benchmarking #17

Job	Run time
set-models	12s
export-models (stories110M) / linux-job	10m 49s
build-llm-demo (tiktoken) / linux-job	31m 9s
build-llm-demo (bpe) / linux-job	29m 20s
upload-models	28s
upload-android-apps	1m 28s
benchmark-on-device (bpe) / mobile-job (android)	5m 43s
	1h 19m 9s

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