* Add XLA torchrun support
* Clarify that currently DDP doesn't work with torch.distributed XLA backend yet
* Enable DDP with torchrun and XLA (now available in PT-XLA 1.13)
* Add check for AWS Neuron availability and AWS Neuron specific compiler flag
* Change the new test's name to TestTrainerDistributedNeuronCore
* Remove "assert" and replace raised exception
* Remove compiler flag as it is optional. If needed, will be another PR.
* Use TORCHELASTIC_RUN_ID to determine whether torchrun is used