Fix `LightningModule.{un,}toggle_model` when only 1 optimizer is used (#12088)

Co-authored-by: Carlos Mocholí <carlossmocholi@gmail.com>

CHANGELOG.md

@@ -666,6 +666,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed the mid-epoch warning call while resuming training ([#11556](https://github.com/PyTorchLightning/pytorch-lightning/pull/11556))
 
 
+- Fixed `LightningModule.{un,}toggle_model` when only 1 optimizer is used ([#12088](https://github.com/PyTorchLightning/pytorch-lightning/pull/12088))
+
+
 - Fixed an issue in `RichProgressbar` to display the metrics logged only on main progress bar ([#11690](https://github.com/PyTorchLightning/pytorch-lightning/pull/11690))
 
 

pytorch_lightning/core/lightning.py

@@ -1384,7 +1384,7 @@ class LightningModule(
         # Iterate over all optimizer parameters to preserve their `requires_grad` information
         # in case these are pre-defined during `configure_optimizers`
         param_requires_grad_state = {}
-        for opt in self.optimizers(use_pl_optimizer=False):
+        for opt in self.trainer.optimizers:
             for group in opt.param_groups:
                 for param in group["params"]:
                     # If a param already appear in param_requires_grad_state, continue
@@ -1408,7 +1408,7 @@ class LightningModule(
         Args:
             optimizer_idx: The index of the optimizer to untoggle.
         """
-        for opt_idx, opt in enumerate(self.optimizers(use_pl_optimizer=False)):
+        for opt_idx, opt in enumerate(self.trainer.optimizers):
             if optimizer_idx != opt_idx:
                 for group in opt.param_groups:
                     for param in group["params"]:

tests/core/test_lightning_module.py

@@ -87,6 +87,23 @@ def test_property_loggers(tmpdir):
     assert model.loggers == [logger]
 
 
+def test_1_optimizer_toggle_model():
+    """Test toggle_model runs when only one optimizer is used."""
+    model = BoringModel()
+    trainer = Mock()
+    model.trainer = trainer
+    params = model.parameters()
+    optimizer = torch.optim.SGD(params, lr=0.1)
+    trainer.optimizers = [optimizer]
+
+    assert not model._param_requires_grad_state
+    # toggle optimizer was failing with a single optimizer
+    model.toggle_optimizer(optimizer, 0)
+    assert model._param_requires_grad_state
+    model.untoggle_optimizer(0)
+    assert not model._param_requires_grad_state
+
+
 def test_toggle_untoggle_2_optimizers_no_shared_parameters(tmpdir):
     class TestModel(BoringModel):
         def training_step(self, batch, batch_idx, optimizer_idx=None):