[Docs] Updating init_process_group docs to indicate correct rank range (#49131)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49131
Users frequently assume the correct range of ranks is 1 ...
`world_size`. This PR udpates the docs to indicate that the correct rank range
users should specify is 0 ... `world_size` - 1.
Test Plan: Rendering and Building Docs
Reviewed By: mrshenli
Differential Revision: D25410532
fbshipit-source-id: fe0f17a4369b533dc98543204a38b8558e68497a
