Update finetune launch command and remove local_rank

[nopperl]

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The readme instructs to use the old torch.distributed.launch command instead of torch.distributed.run, which is incompatible with finetune/finetune.py because it sets the local rank using --local-rank instead of --local_rank, leading to the following error:

finetune.py: error: unrecognized arguments: --local-rank=0

This PR updates the readme to use torch.distributed.run.

Also, it seems like the --local_rank argument is not necessary because it's not used in finetune/finetune.py at all, so I removed it.