Fix output_embeddings get/ set for empty active head (#754)

Fixes #742.

src/adapters/heads/model_mixin.py

@@ -94,6 +94,8 @@ def _init_head_modules(self):
     def get_output_embeddings(self) -> Union[nn.Module, List[nn.Module]]:
         # Only gets the output embeddings for the currently active head
         embeddings = []
+        if not self._active_heads:
+            return None
         for head_name in self._active_heads:
             if head_name in self.heads:
                 head = self.heads[head_name]
@@ -109,6 +111,8 @@ def get_output_embeddings(self) -> Union[nn.Module, List[nn.Module]]:
 
     def set_output_embeddings(self, new_embeddings: Union[nn.Module, List[nn.Module]]):
         # Only sets the output embeddings for the currently active head
+        if not self._active_heads:
+            return
         if not isinstance(new_embeddings, list):
             new_embeddings = [new_embeddings] * len(self._active_heads)
         for head_name, emb in zip(self._active_heads, new_embeddings):

tests/test_adapter_heads.py

@@ -258,6 +258,12 @@ def test_delete_head(self):
         self.assertFalse(name in model.config.prediction_heads)
         self.assertNotEqual(name, model.active_head)
 
+        # add head again
+        self.add_head(model, name)
+        self.assertTrue(name in model.heads)
+        self.assertTrue(name in model.config.prediction_heads)
+        self.assertEqual(name, model.active_head)
+
     def test_adapter_with_head(self):
         model1, model2 = create_twin_models(self.model_class, self.config)