Limit softmax to causally-valid elements in cpu_sdpa #1661

Job	Run time
test-arm / test-backend-linux (arm_tosa_fp, models) / linux-job	31m 45s
test-arm / test-backend-linux (arm_vgf_fp, operators) / linux-job	17m 10s
test-arm / test-backend-linux (arm_tosa_fp, operators) / linux-job	18m 48s
test-arm / test-backend-macos	0s
test-arm / test-backend-linux (arm_vgf_fp, models) / linux-job	26m 58s
test-arm / package-golden-artifacts	46s
	1h 35m 27s

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