Various improvements to `torch.distributed.launch` and `torch.distributed.run` (#60925)

[aivanou]

Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/60925

Pull Request resolved: https://github.com/pytorch/pytorch/pull/60808

• Make torch.distributed.launch restarts to 0
• Remove unnecessary -use_env warning, move -use_env warnings
• Move -use_env warnings to torch.distributed.launch
• Make default log level WARNING
• Add new doc section around transitioning to torch.distributed.run
• Make torch.distributed.launch not use error-propagation
• Set default events handler to null that does not print events to console
• Add reference from torch.distributed.launch to torch.distributed.run
• Set correct preexec function that sends SIGTERM to child processes when parent dies

Issues resolved:

https://github.com/pytorch/pytorch/issues/60716 https://github.com/pytorch/pytorch/issues/60754

Differential Revision: D29413019

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D29413019

[aivanou]

https://github.com/pytorch/pytorch/pull/61294