Update BiRefNet_config.py

Fix the compatibility with transformers >=4.56

BiRefNet_config.py

@@ -5,7 +5,9 @@ class BiRefNetConfig(PretrainedConfig):
     def __init__(
         self,
         bb_pretrained=False,
+        is_encoder_decoder=False,
         **kwargs
     ):
         self.bb_pretrained = bb_pretrained
+        self.is_encoder_decoder = is_encoder_decoder
         super().__init__(**kwargs)