Consider freeze_backbone when building Panoptic Mask RCNN model

PanopticMaskRCNNTask inherits MaskRCNNTask, but didn't implement freeze_backbone.

PiperOrigin-RevId: 475928827

official/projects/panoptic/tasks/panoptic_maskrcnn.py

@@ -57,6 +57,10 @@ class PanopticMaskRCNNTask(maskrcnn.MaskRCNNTask):
         input_specs=input_specs,
         model_config=self.task_config.model,
         l2_regularizer=l2_regularizer)
+
+    if self.task_config.freeze_backbone:
+      model.backbone.trainable = False
+
     return model
 
   def initialize(self, model: tf.keras.Model) -> None: