Frank Gehry does not generally work on a small scale. His buildings swoop and sway and generally defy you to ignore their presence. But the legendary architect is taking his vision into the background this month in upstate New York, where he will make his debut as a set designer for the theatre. Gehry’s set for the Janacek opera Osud will be on display, albeit in a decidedly supporting role, at the SummerScape festival at Bard College.