Fix missing Keras variables shim (horovod#3907)

Signed-off-by: Nicolas Castet <[email protected]>

horovod/_keras/__init__.py

@@ -79,13 +79,15 @@ def __init__(self, **kwargs):
                         scale_local_gradients=scale_local_gradients
                     )
 
-        def variables(self):
-            if _IS_TF2:
-                return super(self.__class__, self).variables()
-            return self.get_weights()
-
         if version.parse(tf.__version__) >= version.parse('2.12.0'):
-            variables = property(variables)
+            @property
+            def variables(self):
+                return CallableList(super(self.__class__, self).variables())
+        else:
+            def variables(self):
+                if _IS_TF2:
+                    return super(self.__class__, self).variables()
+                return self.get_weights()
 
         def register_local_var(self, var):
             """Registers a source/variable as worker local. Horovod will not perform any global
@@ -253,6 +255,13 @@ def apply_gradients(self, *args, **kwargs):
     return cls.from_config(optimizer.get_config())
 
 
+class CallableList(list):
+    """Temporary shim to support both `opt.variables()` and `opt.variables`."""
+
+    def __call__(self):
+        return self
+
+
 def _eval(backend, op_or_result):
     if hvd._executing_eagerly():
         return op_or_result