Fix num_nodes not set for FSDPStrategy (#17438)

(cherry picked from commit 2e5a7f9)

src/lightning/pytorch/CHANGELOG.md

@@ -16,6 +16,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 - Fixed issue where `Model.load_from_checkpoint("checkpoint.ckpt", map_location=map_location)` would always return model on CPU ([#17308](https://github.com/Lightning-AI/lightning/pull/17308))
 
+- Fixed an issue that caused `num_nodes` not to be set correctly for `FSDPStrategy` ([#17438](https://github.com/Lightning-AI/lightning/pull/17438))
 
 
 ## [2.0.1] - 2023-03-30

src/lightning/pytorch/trainer/connectors/accelerator_connector.py

@@ -571,7 +571,7 @@ def _lazy_init_strategy(self) -> None:
             else:
                 self.strategy.parallel_devices = self._parallel_devices
         if hasattr(self.strategy, "num_nodes"):
-            self.strategy._num_nodes = self._num_nodes_flag
+            self.strategy.num_nodes = self._num_nodes_flag
         if hasattr(self.strategy, "_layer_sync"):
             self.strategy._layer_sync = self._layer_sync
         if hasattr(self.strategy, "set_world_ranks"):

tests/tests_pytorch/trainer/connectors/test_accelerator_connector.py

@@ -971,3 +971,17 @@ def _mock_tpu_available(value):
     assert isinstance(connector.strategy.cluster_environment, XLAEnvironment)
     assert connector.strategy._start_method == "fork"
     assert connector.strategy.launcher.is_interactive_compatible
+
+
+@pytest.mark.parametrize(
+    "strategy",
+    [
+        "ddp",
+        "ddp_spawn",
+        pytest.param("deepspeed", marks=RunIf(deepspeed=True)),
+        pytest.param("fsdp", marks=RunIf(min_torch="1.12.0")),
+    ],
+)
+def test_connector_sets_num_nodes(strategy, cuda_count_2):
+    trainer = Trainer(accelerator="cuda", strategy=strategy, devices=2, num_nodes=2)
+    assert trainer.strategy.num_nodes == 2