[tests] switch to torchrun
This PR fixes the following errors in nightly CI tests 
```
FAILED tests/extended/test_trainer_ext.py::TestTrainerExt::test_run_seq2seq_apex
1422
FAILED tests/extended/test_trainer_ext.py::TestTrainerExt::test_run_seq2seq_ddp
1423
FAILED tests/extended/test_trainer_ext.py::TestTrainerExt::test_trainer_log_level_replica_0_base
1424
FAILED tests/extended/test_trainer_ext.py::TestTrainerExt::test_trainer_log_level_replica_1_low
1425
FAILED tests/extended/test_trainer_ext.py::TestTrainerExt::test_trainer_log_level_replica_2_high
1426
FAILED tests/extended/test_trainer_ext.py::TestTrainerExt::test_trainer_log_level_replica_3_mixed
```


by switching from deprecated `distributed.launch` to `distributed.run`
```
:   File "/workspace/transformers/examples/pytorch/translation/run_translation.py", line 664, in <module>
384
stderr:     main()
385
stderr:   File "/workspace/transformers/examples/pytorch/translation/run_translation.py", line 262, in main
386
stderr:     model_args, data_args, training_args = parser.parse_args_into_dataclasses()
387
stderr:   File "/workspace/transformers/src/transformers/hf_argparser.py", line 341, in parse_args_into_dataclasses
388
stderr:     raise ValueError(f"Some specified arguments are not used by the HfArgumentParser: {remaining_args}")
389
stderr: ValueError: Some specified arguments are not used by the HfArgumentParser: ['--local-rank=1']
390
stderr: /opt/conda/lib/python3.8/site-packages/torch/distributed/launch.py:181: FutureWarning: The module torch.distributed.launch is deprecated
391
stderr: and will be removed in future. Use torchrun.
392
stderr: Note that --use-env is set by default in torchrun.
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stderr: If your script expects `--local-rank` argument to be set, please
394
stderr: change it to read from `os.environ['LOCAL_RANK']` instead. See
395
stderr: https://pytorch.org/docs/stable/distributed.html#launch-utility for
396
stderr: further instructions
```

I updated `tests/trainer/test_trainer_distributed.py` while at it.
_The documentation is not available anymore as the PR was closed or merged._<|||||>Unfortunately, conda refuses to install `libstdcxx-ng=12`, and gives a super super long report of conflict packages after 20 or more minutes of examization.

I think we can merge this PR first. And I can try if there is anyway to make to get `GLIBCXX_3.4.30` installed.
@stas00  Does this work for you?<|||||>The `GLIBCXX_3.4.30` is totally unrelated to this issue so let's deal with it separately.