resolve KeyError: 'PDSH_SSH_ARGS_APPEND' (#5318)

when start job with `deepspeed --hostfile hostfile --master_addr
$MASTER_IP --ssh_port 20023 src/train_bash.py `

get error: KeyError: 'PDSH_SSH_ARGS_APPEND' in
https://github.com/microsoft/DeepSpeed/blob/master/deepspeed/launcher/multinode_runner.py#L77

because PDSH_SSH_ARGS_APPEND not in environment.

---------

Co-authored-by: Logan Adams <114770087+loadams@users.noreply.github.com>

Author: Lzhang-hub

deepspeed/launcher/multinode_runner.py

@@ -74,7 +74,8 @@ def name(self):
     def get_cmd(self, environment, active_resources):
         environment['PDSH_RCMD_TYPE'] = 'ssh'
         if self.args.ssh_port is not None:  # only specify ssh port if it is specified
-            environment["PDSH_SSH_ARGS_APPEND"] += f" -p {self.args.ssh_port}"
+            environment["PDSH_SSH_ARGS_APPEND"] = f"{environment.get('PDSH_SSH_ARGS_APPEND', '')} \
+            -p {self.args.ssh_port}"
 
         active_workers = ",".join(active_resources.keys())
         logger.info("Running on the following workers: %s" % active_workers)