Add adapt_checkpoint_hparams hook for customizing checkpoint hyperparameter loading

src/lightning/pytorch/cli.py

@@ -560,6 +560,41 @@ def parse_arguments(self, parser: LightningArgumentParser, args: ArgsType) -> No
         else:
             self.config = parser.parse_args(args)
 
+    def adapt_checkpoint_hparams(self, subcommand: str, checkpoint_hparams: dict[str, Any]) -> dict[str, Any]:
+        """Adapt checkpoint hyperparameters before instantiating the model class.
+
+        This method allows for customization of hyperparameters loaded from a checkpoint when
+        using a different model class than the one used for training. For example, when loading
+        a checkpoint from a TrainingModule to use with an InferenceModule that has different
+        ``__init__`` parameters, you can remove or modify incompatible hyperparameters.
+
+        Args:
+            subcommand: The subcommand being executed (e.g., 'fit', 'validate', 'test', 'predict').
+                This allows you to apply different hyperparameter adaptations depending on the context.
+            checkpoint_hparams: Dictionary of hyperparameters loaded from the checkpoint.
+
+        Returns:
+            Dictionary of adapted hyperparameters to be used for model instantiation.
+
+        Example::
+
+            class MyCLI(LightningCLI):
+                def adapt_checkpoint_hparams(
+                    self, subcommand: str, checkpoint_hparams: dict[str, Any]
+                ) -> dict[str, Any]:
+                    # Only remove training-specific hyperparameters for non-fit subcommands
+                    if subcommand != "fit":
+                        checkpoint_hparams.pop("lr", None)
+                        checkpoint_hparams.pop("weight_decay", None)
+                    return checkpoint_hparams
+
+        Note:
+            If subclass module mode is enabled and ``_class_path`` is present in the checkpoint
+            hyperparameters, you may need to modify it as well to point to your new module class.
+
+        """
+        return checkpoint_hparams
+
     def _parse_ckpt_path(self) -> None:
         """If a checkpoint path is given, parse the hyperparameters from the checkpoint and update the config."""
         if not self.config.get("subcommand"):
@@ -571,6 +606,12 @@ def _parse_ckpt_path(self) -> None:
             hparams.pop("_instantiator", None)
             if not hparams:
                 return
+
+            # Allow customization of checkpoint hyperparameters via adapt_checkpoint_hparams hook
+            hparams = self.adapt_checkpoint_hparams(self.config.subcommand, hparams)
+            if not hparams:
+                return
+
             if "_class_path" in hparams:
                 hparams = {
                     "class_path": hparams.pop("_class_path"),

tests/tests_pytorch/test_cli.py

@@ -495,6 +495,21 @@ def __init__(self, out_dim: int = 2, hidden_dim: int = 2) -> None:
         self.layer = torch.nn.Linear(32, out_dim)
 
 
+class AdaptHparamsModel(BoringModel):
+    """Simple model for testing adapt_checkpoint_hparams hook without dynamic neural network layers.
+
+    This model stores hyperparameters as attributes without creating layers that would cause size mismatches when
+    hyperparameters are changed between fit and predict phases.
+
+    """
+
+    def __init__(self, out_dim: int = 8, hidden_dim: int = 16) -> None:
+        super().__init__()
+        self.save_hyperparameters()
+        self.out_dim = out_dim
+        self.hidden_dim = hidden_dim
+
+
 def test_lightning_cli_ckpt_path_argument_hparams(cleandir):
     class CkptPathCLI(LightningCLI):
         def add_arguments_to_parser(self, parser):
@@ -562,6 +577,62 @@ def add_arguments_to_parser(self, parser):
     assert cli.model.layer.out_features == 4
 
 
+def test_adapt_checkpoint_hparams_hook_pop_keys(cleandir):
+    """Test that the adapt_checkpoint_hparams hook is called and modifications are applied."""
+
+    class AdaptHparamsCLI(LightningCLI):
+        def adapt_checkpoint_hparams(self, subcommand: str, checkpoint_hparams: dict) -> dict:
+            """Remove out_dim and hidden_dim for non-fit subcommands."""
+            if subcommand != "fit":
+                checkpoint_hparams.pop("out_dim", None)
+                checkpoint_hparams.pop("hidden_dim", None)
+            return checkpoint_hparams
+
+    # First, create a checkpoint by running fit
+    cli_args = ["fit", "--model.out_dim=3", "--model.hidden_dim=6", "--trainer.max_epochs=1"]
+    with mock.patch("sys.argv", ["any.py"] + cli_args):
+        cli = AdaptHparamsCLI(AdaptHparamsModel)
+
+    assert cli.config.fit.model.out_dim == 3
+    assert cli.config.fit.model.hidden_dim == 6
+
+    checkpoint_path = next(Path(cli.trainer.log_dir, "checkpoints").glob("*.ckpt"))
+
+    # Test that predict uses adapted hparams (without out_dim and hidden_dim)
+    cli_args = ["predict", f"--ckpt_path={checkpoint_path}", "--model.out_dim=5", "--model.hidden_dim=10"]
+    with mock.patch("sys.argv", ["any.py"] + cli_args):
+        cli = AdaptHparamsCLI(AdaptHparamsModel)
+
+    # Since we removed out_dim and hidden_dim for predict, the CLI values should be used
+    assert cli.config.predict.model.out_dim == 5
+    assert cli.config.predict.model.hidden_dim == 10
+
+
+def test_adapt_checkpoint_hparams_hook_empty_dict(cleandir):
+    """Test that returning empty dict from adapt_checkpoint_hparams disables checkpoint hyperparameter loading."""
+
+    class AdaptHparamsEmptyCLI(LightningCLI):
+        def adapt_checkpoint_hparams(self, subcommand: str, checkpoint_hparams: dict) -> dict:
+            """Disable checkpoint hyperparameter loading."""
+            return {}
+
+    # First, create a checkpoint
+    cli_args = ["fit", "--model.out_dim=3", "--trainer.max_epochs=1"]
+    with mock.patch("sys.argv", ["any.py"] + cli_args):
+        cli = AdaptHparamsEmptyCLI(AdaptHparamsModel)
+
+    checkpoint_path = next(Path(cli.trainer.log_dir, "checkpoints").glob("*.ckpt"))
+
+    # Test that predict uses default values when hook returns empty dict
+    cli_args = ["predict", f"--ckpt_path={checkpoint_path}"]
+    with mock.patch("sys.argv", ["any.py"] + cli_args):
+        cli = AdaptHparamsEmptyCLI(AdaptHparamsModel)
+
+    # Model should use default values (out_dim=8, hidden_dim=16)
+    assert cli.config_init.predict.model.out_dim == 8
+    assert cli.config_init.predict.model.hidden_dim == 16
+
+
 def test_lightning_cli_submodules(cleandir):
     class MainModule(BoringModel):
         def __init__(self, submodule1: LightningModule, submodule2: LightningModule, main_param: int = 1):