Remove the requirement for FSDPStrategy subclasses to only support GPU (#19894)

Author: awaelchli

src/lightning/fabric/connector.py

@@ -429,7 +429,7 @@ def _check_strategy_and_fallback(self) -> None:
                 f" platform. We recommed `Fabric(strategy='ddp_spawn')` instead."
             )
         if (
-            strategy_flag in _FSDP_ALIASES or isinstance(self._strategy_flag, FSDPStrategy)
+            strategy_flag in _FSDP_ALIASES or type(self._strategy_flag) is FSDPStrategy
         ) and self._accelerator_flag not in ("cuda", "gpu"):
             raise ValueError(
                 "You selected the FSDP strategy but FSDP is only available on GPU. Set `Fabric(accelerator='gpu', ...)`"

tests/tests_fabric/test_connector.py

@@ -978,6 +978,16 @@ def test_fsdp_unsupported_on_cpu(_):
     with pytest.raises(ValueError, match="You selected the FSDP strategy but FSDP is only available on GPU"):
         _Connector(accelerator="cpu", strategy="fsdp")
 
+    class FSDPStrategySubclass(FSDPStrategy):
+        pass
+
+    class AcceleratorSubclass(CPUAccelerator):
+        pass
+
+    # we allow subclasses of FSDPStrategy to be used with other accelerators
+    _Connector(accelerator="cpu", strategy=FSDPStrategySubclass())
+    _Connector(accelerator=AcceleratorSubclass(), strategy=FSDPStrategySubclass())
+
 
 def test_connector_defaults_match_fabric_defaults():
     """Test that the default values for the init arguments of Connector match the ones in Fabric."""