Add `cb_multimask_output`

configs/default_hparams.yaml

@@ -1,4 +1,4 @@
-mask_gen_hparams:
+mask_hparams:
   points_per_side: 64
   points_per_batch: 128
   pred_iou_thresh: 0.7
@@ -8,4 +8,5 @@ mask_gen_hparams:
   box_nms_thresh: 0.7
   crop_n_points_downscale_factor: 2
   min_mask_region_area: 25.0
-  use_m2m: true
+  use_m2m: true
+  multimask_output: true