Fix attribute error in SWA when running with Tuner

src/pytorch_lightning/CHANGELOG.md

@@ -216,6 +216,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed an issue with `LightningLite.setup()` not setting the `.device` attribute correctly on the returned wrapper ([#14822](https://github.com/Lightning-AI/lightning/pull/14822))
 
 
+- Fixed an attribute error when running the tuner together with the `StochasticWeightAveraging` callback ([#14836](https://github.com/Lightning-AI/lightning/pull/14836))
+
+
 
 ## [1.7.6] - 2022-09-13
 

src/pytorch_lightning/callbacks/stochastic_weight_avg.py

@@ -123,7 +123,7 @@ def __init__(
         self._avg_fn = avg_fn or self.avg_fn
         self._device = device
         self._model_contains_batch_norm: Optional[bool] = None
-        self._average_model: "pl.LightningModule"
+        self._average_model: Optional["pl.LightningModule"] = None
         self._initialized = False
         self._swa_scheduler: Optional[_LRScheduler] = None
         self._scheduler_state: Optional[Dict] = None
@@ -179,6 +179,7 @@ def on_train_epoch_start(self, trainer: "pl.Trainer", pl_module: "pl.LightningMo
             self._initialized = True
 
             # move average model to request device.
+            assert self._average_model is not None
             self._average_model = self._average_model.to(self._device or pl_module.device)
 
             optimizer = trainer.optimizers[0]
@@ -232,12 +233,14 @@ def on_train_epoch_start(self, trainer: "pl.Trainer", pl_module: "pl.LightningMo
             trainer.current_epoch > self._latest_update_epoch
         ):
             assert self.n_averaged is not None
+            assert self._average_model is not None
             self.update_parameters(self._average_model, pl_module, self.n_averaged, self._avg_fn)
             self._latest_update_epoch = trainer.current_epoch
 
         # Note: No > here in case the callback is saved with the model and training continues
         if trainer.current_epoch == self.swa_end + 1:
             # Transfer weights from average model to pl_module
+            assert self._average_model is not None
             self.transfer_weights(self._average_model, pl_module)
 
             # Reset BatchNorm for update
@@ -266,6 +269,7 @@ def on_train_end(self, trainer: "pl.Trainer", pl_module: "pl.LightningModule") -
             self.reset_momenta()
         elif trainer.current_epoch - 1 == self.swa_end:
             # Last SWA epoch. Transfer weights from average model to pl_module
+            assert self._average_model is not None
             self.transfer_weights(self._average_model, pl_module)
 
     @staticmethod

tests/tests_pytorch/callbacks/test_stochastic_weight_avg.py

@@ -32,6 +32,22 @@
 from tests_pytorch.helpers.runif import RunIf
 
 
+def test_swa_callback_initial_state():
+    swa = StochasticWeightAveraging(
+        swa_lrs=0.01,
+        swa_epoch_start=0.1,
+        annealing_epochs=1,
+        annealing_strategy="linear",
+        avg_fn=sum,
+    )
+    assert swa._swa_lrs == 0.01
+    assert swa._swa_epoch_start == 0.1
+    assert swa._annealing_epochs == 1
+    assert swa._annealing_strategy == "linear"
+    assert swa._avg_fn == sum
+    assert swa._average_model is None
+
+
 class SwaTestModel(BoringModel):
     def __init__(
         self, batchnorm: bool = True, interval: str = "epoch", iterable_dataset: bool = False, crash_on_epoch=None