Cortex-M backend: fix avg_pool2d substitution function signature (#20… #631

Job	Run time
test-webgpu / test-backend-linux (webgpu, models) / linux-job	16m 49s
test-webgpu / test-backend-linux (webgpu, operators) / linux-job	36m 3s
test-webgpu / test-backend-macos	-7s
test-webgpu / package-golden-artifacts	1m 35s
	54m 20s

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