Update hub_mixin.py (#909)

Fix the error caused by pop operation on non-existing key in dict.

segmentation_models_pytorch/base/hub_mixin.py

@@ -120,7 +120,7 @@ def save_pretrained(
         finally:
             # delete the additional attributes
             self._del_attrs(["save_directory", "metrics", "dataset"])
-            self._hub_mixin_config.pop("_model_class")
+            self._hub_mixin_config.pop("_model_class", None)
 
         return result