Add Half (float16) support to slim ScalarType enum (#18959) #2573

Job	Run time
test-xnnpack / test-backend-linux (xnnpack, models) / linux-job	10m 46s
test-xnnpack / test-backend-linux (xnnpack, operators) / linux-job	10m 20s
test-xnnpack / test-backend-macos	0s
test-xnnpack / package-golden-artifacts	1s
	21m 7s

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