Retrieves ads for each of the given placements. Each placement maps to a typed response object. Banner placements return a single flattened ad; multi-banner placements return an array of ads ordered by auction rank. If no ad is available for a placement, the placement will be missing from the response. Placements are specified as repeated placements query parameters. Optionally, num_results[{id}]=N can be used to request a specific number of ads for a placement. When omitted, the placement's configured maximum is used.