docs: document PyTorch backend #4770

Job	Run time
Build source distribution	26s
determine-arm64-runner	1s
Build wheels for cp311-manylinux_x86_64	5m 6s
Build wheels for cp311-manylinux_x86_64	5m 38s
Build wheels for cp311-macosx_x86_64	5m 44s
Build wheels for cp311-macosx_arm64	7m 43s
Build wheels for cp311-win_amd64	4m 54s
Build wheels for cp310-manylinux_aarch64	4m 17s
build_docker (12)	3m 39s
build_pypi_index	6s
build_docker (_cu11, 11)	4m 24s
upload_pypi	0s
deploy_pypi_index	0s
Pass testing build wheels	2s
	42m 0s

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