Minor Fabric backward refactor (#17433)

src/lightning/fabric/fabric.py

@@ -363,7 +363,7 @@ class Fabric:
                 # requires to attach the current `DeepSpeedEngine` for the `_FabricOptimizer.step` call.
                 self._strategy._deepspeed_engine = module
 
-        self._precision.backward(tensor, module, *args, **kwargs)
+        self._strategy.backward(tensor, module, *args, **kwargs)
 
     def clip_gradients(
         self,

tests/tests_fabric/test_fabric.py

@@ -568,10 +568,10 @@ def test_rank_properties():
 def test_backward():
     """Test that backward() calls into the precision plugin."""
     fabric = Fabric()
-    fabric._precision = Mock(spec=Precision)
+    fabric._strategy = Mock(spec=Precision)
     loss = Mock()
     fabric.backward(loss, "arg", keyword="kwarg")
-    fabric._precision.backward.assert_called_with(loss, None, "arg", keyword="kwarg")
+    fabric._strategy.backward.assert_called_with(loss, None, "arg", keyword="kwarg")
 
 
 @RunIf(deepspeed=True, mps=False)