[doc] reorg dist init and non-init functions (#52976)
Summary:
This PR proposes to improve the distributed doc:
* [x] putting the init functions together
* [x] moving post-init functions into their own sub-section as they are only available after init and moving that group to after all init sub-sections
If this is too much, could we at least put these 2 functions together:
```
.. autofunction:: init_process_group
.. autofunction:: is_initialized
```
as they are interconnected. and the other functions are not alphabetically sorted in the first place.
Thank you.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52976
Reviewed By: albanD
Differential Revision: D26993933
Pulled By: mrshenli
fbshipit-source-id: 7cacbe28172ebb5849135567b1d734870b49de77
