TransformerBlock: support attention skips #1483

Job	Run time
test-executorch-cuda-build-12.6 / linux-job	19m 19s
test-executorch-cuda-build-13.0 / linux-job	19m 16s
test-executorch-cuda-build-12.8 / linux-job	20m 14s
check-all-cuda-builds	3s
	58m 52s

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