Use torchrun for dynamo/distributed.py (#89149)
Mainly wanted to confirm torchrun works fine with dynamo/ddp,
but it is also a better system than manually launching processes.
Partially addresses issue #1779
New run commands
------------
single process:
python benchmarks/dynamo/distributed.py [args]
multi-gpu (e.g. 2 gpu on one host):
torchrun --nproc_per_node 2 benchmarks/dynamo/distributed.py [args]
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89149
Approved by: https://github.com/aazzolini
