Added default backend value

[vfdev-5]

Description

Follow-up PR to #203, adds default backend value.
Otherwise we see an incomplete command:

python -m torch.distributed.launch \
  --nproc_per_node 2 \
  --use_env main.py \
  --backend 

cc @sayantan1410

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[sayantan1410]

Thanks for mentioning I forgot about the default case while making the PR.