[19/n][torch/elastic][upstream] Replace pytorch.distributed.launch with torchelastic launcher (#56037)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56037
The diff introduces new `torch.distributed.elastic_launch` and removes internals of `torch.distributed.launch` keeping backwards compatibility.
Since torchelastic and torch.launch are not fully compatible due to `--use_env` arg, the `torch.distributed.launch` deprecation is going to be iterative: as part of pytorch 1.9 we are going to deprecate it, and in the following releases we will remove `torch.distributed.launch`
The diff leaves `torchelastic.distributed.launch` module, and the follow up diffs will migrate the users form `torchelastic.distributed.launch` to `torch.distributed.elastic_launch`
Test Plan: buck test mode/dev-nosan //pytorch/elastic/torchelastic/distributed/...
Reviewed By: cbalioglu
Differential Revision: D27753803
fbshipit-source-id: 5f24bcfdcb70356f0787b11f6cb9479f3515fb47
