fix regression flash attention (#1695) #2755

Job	Run time
build-and-test-cpp-aarch64 (openblas)	4m 1s
build-and-push-docker-images	1h 49m 46s
build-python-wheels (ubuntu-20.04, auto64)	2h 2m 37s
build-and-test-cpp-x86_64 (ubuntu-20.04, mkl)	5m 36s
build-python-wheels (macos-11, auto64)	35m 34s
build-and-test-cpp-x86_64 (ubuntu-20.04, dnnl)	5m 39s
build-python-wheels (windows-2019, auto64)	5h 34m 41s
check-python-style	9s
build-and-test-cpp-x86_64 (macos-11, mkl)	10m 5s
build-python-wheels (ubuntu-20.04, aarch64)	1h 16m 26s
build-and-test-cpp-x86_64 (macos-11, dnnl)	9m 50s
build-python-wheels (macos-11, arm64)	13m 40s
test-python-wheels (ubuntu-20.04)	7m 40s
build-and-deploy-docs	1m 4s
test-python-wheels (macos-11)	5m 38s
test-python-wheels (windows-2019)	4m 40s
publish-python-wheels-on-pypi	0s
	12h 27m 6s

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