Keep FFN output layer in float32 for T5 models #2310

Job	Run time
build-and-push-docker-images	41m 24s
build-and-test-cpp-aarch64 (openblas)	5m 55s
build-python-wheels (ubuntu-20.04, auto64)	39m 9s
build-python-wheels (macos-11, auto64)	34m 56s
build-python-wheels (windows-2019, auto64)	1h 3m 31s
build-python-wheels (ubuntu-20.04, aarch64)	1h 49m 12s
build-python-wheels (macos-11, arm64)	9m 47s
build-and-test-cpp-x86_64 (ubuntu-20.04, mkl)	6m 50s
build-and-test-cpp-x86_64 (ubuntu-20.04, dnnl)	6m 12s
build-and-test-cpp-x86_64 (macos-11, mkl)	10m 43s
build-and-test-cpp-x86_64 (macos-11, dnnl)	9m 14s
check-python-style	10s
build-and-deploy-docs	43s
test-python-wheels (ubuntu-20.04)	13m 7s
test-python-wheels (macos-11)	7m 37s
test-python-wheels (windows-2019)	3m 35s
publish-python-wheels-on-pypi	0s
	6h 2m 5s

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