layernorm support #5680

Job	Run time
apple (aarch64-apple-ios)	1m 51s
linux (raspbian)	7m 16s
apple (aarch64-apple-darwin)	5m 52s
linux (aarch64-unknown-linux-gnu)	12m 8s
linux (armv6vfp-unknown-linux-gnueabihf)	10m 23s
linux (armv7-unknown-linux-gnueabihf)	10m 10s
linux (aarch64-unknown-linux-musl)	13m 14s
linux (armv7-unknown-linux-musl)	10m 44s
linux (aarch64-linux-android)	1m 27s
linux (armv7-linux-androideabi)	1m 26s
linux (i686-linux-android)	1m 19s
linux (x86_64-linux-android)	1m 20s
linux (wasm32-unknown-unknown)	40s
linux (wasm32-wasi)	33s
	1h 18m 23s

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