Small changes to improve blackwell_fmha_test.py (#4896) #18756

Job	Run time
build-macos (macos-13, static)	25m 52s
build-windows (windows-2022, static)	12m 20s
build-macos (macos-13, shared)	19m 57s
build-windows (windows-2022, shared)	12m 11s
build-bazel (x86, linux.12xlarge, gcc)	10m 1s
build-linux (arm, linux.arm64.m7g.4xlarge, static, gcc, 14.1.0)	1h 21m 37s
build-linux (x86, linux.12xlarge, shared, clang, 16.0.6)	19m 54s
build-linux (x86, linux.12xlarge, static, gcc, 14.1.0)	26m 21s
build-linux (arm, linux.arm64.m7g.4xlarge, static, clang, 16.0.6)	32m 52s
build-linux (x86, linux.12xlarge, shared, gcc, 14.1.0)	26m 29s
build-linux (arm, linux.arm64.m7g.4xlarge, shared, gcc, 14.1.0)	1h 20m 20s
build-linux (x86, linux.12xlarge, shared, gcc, 9.5.0)	25m 46s
build-linux (x86, linux.12xlarge, static, gcc, 9.5.0)	25m 59s
build-linux (x86, linux.12xlarge, static, clang, 16.0.6)	20m 4s
build-linux (arm, linux.arm64.m7g.4xlarge, shared, clang, 16.0.6)	32m 57s
build-bazel (x86, linux.12xlarge, clang)	9m 29s
	7h 42m 9s

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