Allow callbacks to be restored not just during training (#20403)

* Allow callbacks to be restored not just during training

* add test case

* test test case failure

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* fix test case

---------

Co-authored-by: Alan Chu <alanchu@Alans-Air.lan>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Luca Antiga <luca.antiga@gmail.com>

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

@@ -397,9 +397,7 @@ class _CheckpointConnector:
         self.resume_start(checkpoint_path)
         self.restore_model()
         self.restore_datamodule()
-        if self.trainer.state.fn == TrainerFn.FITTING:
-            # restore callback states
-            self.restore_callbacks()
+        self.restore_callbacks()
 
     def dump_checkpoint(self, weights_only: bool = False) -> dict:
         """Creating a model checkpoint dictionary object from various component states.

tests/tests_pytorch/trainer/connectors/test_checkpoint_connector.py

@@ -18,7 +18,7 @@ from unittest.mock import ANY, Mock
 import pytest
 import torch
 from lightning.pytorch import Trainer
-from lightning.pytorch.callbacks import ModelCheckpoint
+from lightning.pytorch.callbacks import Callback, ModelCheckpoint
 from lightning.pytorch.demos.boring_classes import BoringModel
 from lightning.pytorch.trainer.states import TrainerFn
 from lightning.pytorch.utilities.migration.utils import _set_version
@@ -234,3 +234,53 @@ def test_strict_loading(strict_loading, expected, tmp_path):
     trainer = Trainer(default_root_dir=tmp_path, barebones=True, max_steps=2)
     trainer.fit(model, ckpt_path=(tmp_path / "checkpoint.ckpt"))
     model.load_state_dict.assert_called_once_with(ANY, strict=expected)
+
+
+@pytest.mark.parametrize("trainer_fn", ["validate", "test", "predict"])
+def test_restore_callbacks_in_non_fit_phases(tmp_path, trainer_fn):
+    """Test that callbacks are properly restored in non-fit phases."""
+
+    class TestCallback(Callback):
+        def __init__(self):
+            self.restored = False
+
+        def on_load_checkpoint(self, trainer, pl_module, checkpoint):
+            if "callbacks" in checkpoint:
+                callback_state = checkpoint["callbacks"][self.__class__.__name__]
+                self.restored = callback_state["restored"]
+
+        def state_dict(self):
+            return {"restored": self.restored}
+
+        def on_save_checkpoint(self, trainer, pl_module, checkpoint):
+            checkpoint["callbacks"] = checkpoint.get("callbacks", {})
+            checkpoint["callbacks"][self.__class__.__name__] = self.state_dict()
+
+    # First create and train a model with the callback
+    callback = TestCallback()
+    model = BoringModel()
+    trainer = Trainer(default_root_dir=tmp_path, callbacks=[callback], max_steps=1)
+    trainer.fit(model)
+
+    # Set the callback state to True before saving
+    callback.restored = True
+    ckpt_path = tmp_path / "checkpoint.ckpt"
+    trainer.save_checkpoint(ckpt_path)
+
+    # Now create new instances and test restoration
+    new_callback = TestCallback()
+    new_model = BoringModel()
+    assert not new_callback.restored  # Should start False
+
+    new_trainer = Trainer(default_root_dir=tmp_path, callbacks=[new_callback])
+
+    # Connect the model and restore callbacks before evaluation
+    new_trainer.strategy.connect(new_model)
+    new_trainer._checkpoint_connector.resume_start(ckpt_path)
+    new_trainer._checkpoint_connector.restore_callbacks()
+
+    # Run the evaluation phase (validate/test/predict)
+    fn = getattr(new_trainer, trainer_fn)
+    fn(new_model, ckpt_path=ckpt_path)
+
+    assert new_callback.restored  # Should be True after loading the checkpoint