Limit softmax to causally-valid elements in cpu_sdpa #1548

Job	Run time
test-coreml / test-backend-macos (coreml, operators) / macos-job	16m 34s
test-coreml / test-backend-macos (coreml, models) / macos-job	19m 0s
test-coreml / test-backend-linux	-1s
test-coreml / package-golden-artifacts	-1s
	35m 32s

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