(torch.distributed) Add torch.distributed.is_torchelastic_launched() util method...

(torch.distributed) Add torch.distributed.is_torchelastic_launched() util method + make init_method=tcp:// compatible with torchelastic (#63910)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63910

Addresses the current issue that `init_method=tcp://` is not compatible with `torch.distributed.run` and `torch.distributed.launch`. When running with a training script that initializes the process group with `init_method=tcp://localhost:$port` as such:

```
$ python -u -m torch.distributed.run --max_restarts 0 --nproc_per_node 1 --nnodes 1 --master_addr $(hostname) --master_port 6000 ~/tmp/test.py
```

An `Address in use` error is raised since the training script tries to create a TCPStore on port 6000, which is already taken since the elastic agent is already running a TCPStore on that port.

For details see: https://github.com/pytorch/pytorch/issues/63874.

This change does a couple of things:

1. Adds `is_torchelastic_launched()` check function that users can use in the training scripts to see whether the script is launched via torchelastic.
1. Update the `torch.distributed` docs page to include the new `is_torchelastic_launched()` function.
1. Makes `init_method=tcp://` torchelastic compatible by modifying `_tcp_rendezvous_handler` in `torch.distributed.rendezvous` (this is NOT the elastic rendezvous, it is the old rendezvous module which is slotted for deprecation in future releases) to check `is_torchelastic_launched()` AND `torchelastic_use_agent_store()` and if so, only create TCPStore clients (no daemons, not even for rank 0).
1. Adds a bunch of unittests to cover the different code paths

NOTE: the issue mentions that we should fail-fast with an assertion on `init_method!=env://` when `is_torchelastic_launched()` is `True`. There are three registered init_methods in pytorch: env://, tcp://, file://. Since this diff makes tcp:// compatible with torchelastic and I've validated that file is compatible with torchelastic. There is no need to add assertions. I did update the docs to point out that env:// is the RECOMMENDED init_method. We should probably deprecate the other init_methods in the future but this is out of scope for this issue.

Test Plan: Unittests.

Reviewed By: cbalioglu

Differential Revision: D30529984

fbshipit-source-id: 267aea6d4dad73eb14a2680ac921f210ff547cc5