Restore ScalelessRMSNorm hand-rolled forward for eager numerical parity #5630

Job	Run time
test-arm / test-backend-linux (arm_tosa_fp, operators) / linux-job	23m 27s
test-arm / test-backend-linux (arm_vgf_fp, models) / linux-job	18m 46s
test-arm / test-backend-linux (arm_tosa_fp, models) / linux-job	18m 0s
test-arm / test-backend-linux (arm_vgf_fp, operators) / linux-job	22m 55s
test-arm / test-backend-macos	0s
test-arm / package-golden-artifacts	46s
	1h 23m 54s

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