Flash attention #2709

Job	Run time
build-and-test-cpp-aarch64 (openblas)	2m 57s
build-and-test-cpp-x86_64 (ubuntu-20.04, mkl)	2m 15s
build-and-push-docker-images	38m 10s
build-and-test-cpp-x86_64 (ubuntu-20.04, dnnl)	2m 1s
build-python-wheels (ubuntu-20.04, auto64)	2m 31s
check-python-style	11s
build-and-test-cpp-x86_64 (macos-11, mkl)	2m 16s
build-python-wheels (macos-11, auto64)	2m 29s
build-and-test-cpp-x86_64 (macos-11, dnnl)	2m 17s
build-python-wheels (windows-2019, auto64)	2m 31s
build-python-wheels (ubuntu-20.04, aarch64)	2m 30s
build-python-wheels (macos-11, arm64)	2m 10s
test-python-wheels	0s
build-and-deploy-docs	0s
publish-python-wheels-on-pypi	0s
	1h 2m 18s

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