[Feature] flatten and unflatten as decorators #2617

Job	Run time
build-wheel-linux (3.8, cp38-cp38, --extra-index-url https://download.pytorch.org/whl/cpu, "['cpu...	1m 6s
build-wheel-windows (3.8, 3.8)	1m 51s
build-wheel-linux (3.9, cp39-cp39, --extra-index-url https://download.pytorch.org/whl/cpu, "['cpu...	1m 24s
build-wheel-windows (3.9, 3.9)	3m 3s
build-wheel-linux (3.10, cp310-cp310, --extra-index-url https://download.pytorch.org/whl/cpu, "['...	1m 8s
build-wheel-windows (3.10, 3.10.3)	4m 11s
build-wheel-linux (3.11, cp311-cp311, --extra-index-url https://download.pytorch.org/whl/cpu, "['...	1m 4s
build-wheel-windows (3.11, 3.11)	3m 2s
test-wheel (linux, ubuntu-20.04, 3.8)	43s
test-wheel (linux, ubuntu-20.04, 3.9)	53s
test-wheel (linux, ubuntu-20.04, 3.10)	55s
test-wheel (linux, ubuntu-20.04, 3.11)	48s
test-wheel-windows (3.8)	2m 56s
test-wheel-windows (3.9)	3m 47s
test-wheel-windows (3.10)	3m 43s
test-wheel-windows (3.11)	2m 51s
	33m 25s

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