[autoscaler] Add tmux support for attach and exec (#2907)

Adds a tmux flag that can be used to support background execution of experiments. Cannot be used together with screen. Seems to be useful feature that has shown up with different users.

doc/source/autoscaling.rst

@@ -70,6 +70,9 @@ You can use ``ray exec`` to conveniently run commands on clusters. Note that scr
     $ ray exec cluster.yaml 'echo "hello world"' \
         --start --stop --cluster-name experiment-1
 
+    # Run a command in a detached tmux session
+    $ ray exec cluster.yaml 'echo "hello world"' --tmux
+
     # Run a command in a screen (experimental)
     $ ray exec cluster.yaml 'echo "hello world"' --screen
 
@@ -86,6 +89,10 @@ You can use ``ray attach`` to attach to an interactive console on the cluster.
     # Open a screen on a new cluster called 'session-1'
     $ ray attach cluster.yaml --start --cluster-name=session-1
 
+    # Attach to tmux session on cluster (creates a new one if none available)
+    $ ray attach cluster.yaml --tmux
+
+
 Port-forwarding applications
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