Fix compute_mask for Birectional with return_state=True

Fix `compute_mask` to properly support `return_state` introduced in Birectional with #8977

keras/layers/wrappers.py

@@ -490,11 +490,20 @@ def build(self, input_shape):
     def compute_mask(self, inputs, mask):
         if self.return_sequences:
             if not self.merge_mode:
-                return [mask, mask]
+                output_mask = [mask, mask]
             else:
-                return mask
+                output_mask = mask
         else:
-            return None
+            output_mask = None
+
+        if self.return_state:
+            states = self.forward_layer.states
+            state_mask = [None for _ in states]
+            if isinstance(output_mask, list):
+                return output_mask + state_mask * 2
+            return [output_mask] + state_mask * 2
+
+        return output_mask
 
     @property
     def trainable_weights(self):