Add named optional parameters to smoketest.sh

[JamesKunstle]

This is a little confusing. We have FSDP as the default, so I can use smoketest.sh as-is. But if I want to set the number of GPUs, do I also have to specify FSDP? Since it is the "second positional argument"? Will it break if I just do smoketest.sh 8? We may want to instead just have like --backend/-b and --num-gpus/-n

Originally posted by @Maxusmusti in #280 (comment)