When Diane Retallack takes the stage, she usually turns her back to the audience. Her focus is on the performers before her — members of the musical groups she directs. After 32 years as artistic director and conductor of Eugene Concert Choir, she’s comfortable in that position, blending into the scene without distracting from the featured performers. But last week, Retallack, 63, was stepping out on a less familiar stage as a contestant in America’s largest ballroom dance co