Fix AttentionInterface following feedback (#37010)

* up

* typo

* update doc

* Update attention_interface.md

src/transformers/modeling_utils.py

@@ -5917,7 +5917,7 @@ class AttentionInterface(MutableMapping):
     """
     Dict-like object keeping track of allowed attention functions. You can easily add a new attention function
     with a call to `register()`. If a model needs to locally overwrite an existing attention function, say `sdpa`,
-    it needs to declare a new instance of this class inside the `modeling.py`, and declare it on that instance.
+    it needs to declare a new instance of this class inside the `modeling_<model>.py`, and declare it on that instance.
     """
 
     # Class instance object, so that a call to `register` can be reflected into all other files correctly, even if
@@ -5946,7 +5946,7 @@ class AttentionInterface(MutableMapping):
 
     def __iter__(self):
         # Ensure we use all keys, with the overwritten ones on top
-        return iter(self._global_mapping.update(self._local_mapping))
+        return iter({**self._global_mapping, **self._local_mapping})
 
     def __len__(self):
         return len(self._global_mapping.keys() | self._local_mapping.keys())
@@ -5956,7 +5956,7 @@ class AttentionInterface(MutableMapping):
         cls._global_mapping.update({key: value})
 
     def valid_keys(self) -> List[str]:
-        return list(self._global_mapping.keys() | self._local_mapping.keys())
+        return list(self.keys())
 
 
 # Global AttentionInterface shared by all models which do not need to overwrite any of the existing ones