Apple

Add fast path for contiguous innermost-dim reduction in mean.out (#18… #19916

Job	Run time
set-version	7s
build-demo-ios / macos-job	43m 42s
build-benchmark-app / macos-job	1h 0m 3s
build-frameworks-ios / macos-job	58m 9s
upload-demo-ios	48s
test-demo-ios / mobile-job (ios)	13m 13s
upload-frameworks-ios	1m 6s
	2h 57m 8s

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