fix autodoc for torch.distributed.launch (#40963)
Summary:
The doc for `torch.distributed.launch` is missing since v1.2.0 (see issue https://github.com/pytorch/pytorch/issues/36386) because PR https://github.com/pytorch/pytorch/issues/22501 added some imports at the first line.
https://github.com/pytorch/pytorch/blob/542ac74987141c81b6326b9e7d5c6a1d00fc3701/torch/distributed/launch.py#L1-L5
I move it below the docstring to make the autodoc in Sphinx work normally.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40963
Differential Revision: D22380816
Pulled By: mrshenli
fbshipit-source-id: ee8406785b9a198bbf3fc65e589854379179496f
