fix softmax masking #10117

Job	Run time
build (3.8, x64, ubuntu)	1h 32m 49s
build (3.8, x64, windows)	1h 32m 32s
build (3.11, x64, ubuntu)	1h 20m 11s
build (3.11, x64, macos)	36m 33s
build (3.11, x64, windows)	1h 34m 51s
build (3.12, x64, ubuntu)	1h 18m 10s
build (3.12, x64, macos)	32m 6s
build (3.12, x64, windows)	1h 33m 15s
build (3.9, macos, --cov=psyneulink)	47m 44s
build (3.9, ubuntu, --forked -m "not llvm")	39m 6s
build (3.9, x86, windows)	1h 35m 17s
build (3.10, ubuntu, --fp-precision=fp32)	1h 18m 4s
build (3.10, macos, -m benchmark --benchmark-enable --benchmark-only --benchmark-min-rounds=2 --b...	26m 12s
build (3.8, macos, min)	37m 5s
	15h 23m 55s

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