[APS IR] Minfor fix - use GetAttrKey in get_keystr to match with flat args path in unflatten (pytorch#161453)

Summary: While passing path info to [_check_input_constraints_for_graph](https://www.internalfb.com/code/fbsource/[6b5b2dc35902a26ce265e3c0ae5189a3faba1d38]/fbcode/caffe2/torch/export/unflatten.py?lines=594), GetAttrKey is used to specify path str. To match with that get_keystr should also use GetAttrKey.

Test Plan:
Existing tests

```
buck run mode/opt caffe2/test:test_export -- -r unflatten
```

```
Ran 413 tests in 204.533s

OK (skipped=1, expected failures=13)
```

Rollback Plan:

Differential Revision: D80984083

Pull Request resolved: pytorch#161453
Approved by: https://github.com/tugsbayasgalan

Author: malaybag

torch/_export/utils.py

@@ -331,7 +331,7 @@ def get_keystr(key_path: KeyPath) -> str:
         return f"*args{keystr(key_path[1:])}"
     else:
         kwarg_key = key_path[1]
-        assert isinstance(kwarg_key, MappingKey)
+        assert isinstance(kwarg_key, GetAttrKey)
         name = str(kwarg_key)[1:-1]  # get rid of the enclosed []
         return f"{name}{keystr(key_path[2:])}"
 

torch/export/unflatten.py

@@ -134,6 +134,11 @@ class _SubmoduleBase:
     _ty: Optional[str]
 
     def type_name(self) -> Optional[str]:
+        """
+        Subclass of this class - InterpreterModule, InterpreterModuleDispatcher, represents
+        corresponding model in eager model. To get this type information for those modules
+        in eager model we need to use this method.
+        """
         return self._ty