fix regression flash attention #2754

Job	Run time
build-and-push-docker-images	1h 51m 4s
build-and-test-cpp-x86_64 (ubuntu-20.04, mkl)	5m 39s
build-and-test-cpp-x86_64 (ubuntu-20.04, dnnl)	5m 20s
build-python-wheels (ubuntu-20.04, auto64)	2h 4m 21s
build-and-test-cpp-aarch64 (openblas)	4m 6s
build-python-wheels (macos-11, auto64)	55m 28s
build-and-test-cpp-x86_64 (macos-11, mkl)	10m 21s
check-python-style	8s
build-python-wheels (windows-2019, auto64)	5h 28m 9s
build-and-test-cpp-x86_64 (macos-11, dnnl)	10m 34s
build-python-wheels (ubuntu-20.04, aarch64)	1h 15m 52s
build-python-wheels (macos-11, arm64)	16m 4s
build-and-deploy-docs	1m 2s
test-python-wheels (ubuntu-20.04)	9m 36s
test-python-wheels (macos-11)	4m 36s
test-python-wheels (windows-2019)	4m 32s
publish-python-wheels-on-pypi	0s
	12h 46m 52s

Provide feedback