Remove warning on no_backward_sync with XLA strategy (#17761)

(cherry picked from commit f3c49b8)

Author: carmocca

src/lightning/fabric/CHANGELOG.md

@@ -11,6 +11,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed model parameters getting shared between processes when running with `strategy="ddp_spawn"` and `accelerator="cpu"`; this has a necessary memory impact, as parameters are replicated for each process now ([#18238](https://github.com/Lightning-AI/lightning/pull/18238))
 
 
+- Removed false positive warning when using `fabric.no_backward_sync` with XLA strategies ([#17761](https://github.com/Lightning-AI/lightning/pull/17761))
+
+
 ## [2.0.7] - 2023-08-14
 
 ### Changed

src/lightning/fabric/fabric.py

@@ -581,7 +581,7 @@ def no_backward_sync(self, module: _FabricModule, enabled: bool = True) -> Gener
                 "You need to set up the model first before you can call `self.no_backward_sync()`:"
                 " `model = self.setup(model, ...)`"
             )
-        if not enabled or isinstance(self._strategy, SingleDeviceStrategy):
+        if not enabled or isinstance(self._strategy, (SingleDeviceStrategy, XLAStrategy)):
             context = nullcontext()
         elif self._strategy._backward_sync_control is None:
             rank_zero_warn(

tests/tests_fabric/test_fabric.py

@@ -625,14 +625,19 @@ def test_no_backward_sync():
     with fabric.no_backward_sync(model):
         pass
     fabric._strategy._backward_sync_control.no_backward_sync.assert_not_called()
+    # same for XLA
+    fabric._strategy = Mock(spec=XLAStrategy, _backward_sync_control=MagicMock())
+    with fabric.no_backward_sync(model):
+        pass
+    fabric._strategy._backward_sync_control.no_backward_sync.assert_not_called()
 
     # pretend that the strategy supports skipping backward sync
     fabric._strategy = Mock(_backward_sync_control=MagicMock())
     # disabling the context manager makes it a no-op
     with fabric.no_backward_sync(model, enabled=False):
         pass
     fabric._strategy._backward_sync_control.no_backward_sync.assert_not_called()
-    # when enabld, the wrapped module gets passed down
+    # when enabled, the wrapped module gets passed down
     with fabric.no_backward_sync(model):
         pass
     fabric._strategy._backward_sync_control.no_backward_sync.assert_called_once_with(model._forward_module)