For a distributed setting you should provide the hostname of one of your machines:
rdzv_endpoint="node0.example.com" rdzv_backend="c10d"

The example that Howard gave was meant to be run on a single machine with two worker processes. localhost:0 simply means use a random port on the local machine for the coordination of the nodes. Since you only have one node (that runs two workers) specifying localhost as an endpoint worked in that example. However if you want to run your training on more than one machine (a.k.a. node) the endpoint should be a hostname/FQDN that is reachable by all machines.