Add state_dict/load_state_dict support for MultiDataloader (#452)

Summary:
Pull Request resolved: #452

As title - MultiDataloader now supports the stateful protocol for checkpoint saving/loading. The dataloader generates a state dict based on the names of `individual_dataloaders` and if the underlying dataloaders themselves are Stateful.

Reviewed By: daniellepintz

Differential Revision: D47409954

fbshipit-source-id: d3700c5672ce950e403438cfb2b28f0c1ff11fbf

tests/utils/data/test_multi_dataloader.py

@@ -8,7 +8,7 @@
 import random
 import unittest
 from collections import Counter
-from typing import Any, Dict, List, Mapping
+from typing import Any, Dict, Iterator, List, Mapping
 
 import torch
 from torch.utils.data import DataLoader, Dataset
@@ -427,3 +427,74 @@ def test_in_order_with_repetitions(self) -> None:
         # Raises StopIteration after all exhausted
         with self.assertRaises(StopIteration):
             batch = next(multi_dataloader)
+
+    def test_state_dict_load_state_dict(self) -> None:
+        class DummyIterable:
+            def __init__(self, vals: List[int]) -> None:
+                self.vals = vals
+                # Start at -1 since an iterator is generated when the MultiDataLoader is constructed
+                # while checking for missing data
+                self.iter_count = -1
+
+            def __iter__(self) -> Iterator[int]:
+                self.iter_count += 1
+                return iter(self.vals)
+
+            def state_dict(self) -> Dict[str, Any]:
+                return {"iter_count": self.iter_count}
+
+            def load_state_dict(self, state_dict: Dict[str, Any]) -> None:
+                self.iter_count = state_dict["iter_count"]
+
+        iterable_1 = DummyIterable([1, 2, 3])
+        iterable_2 = DummyIterable([4, 5, 6])
+        # Add an iterable which does not implement the stateful protocol
+        iterable_3 = [7, 8, 9]
+
+        multi_dataloader = MultiDataLoader(
+            {"foo": iterable_1, "bar": iterable_2, "baz": iterable_3}, InOrder()
+        )
+
+        # Generate state dict from initial state
+        original_state_dict = multi_dataloader.state_dict()
+
+        # Confirm keys are appropriately set
+        self.assertIn("foo", original_state_dict)
+        self.assertIn("iter_count", original_state_dict["foo"])
+        self.assertEqual(0, original_state_dict["foo"]["iter_count"])
+        self.assertIn("bar", original_state_dict)
+        self.assertIn("iter_count", original_state_dict["bar"])
+        self.assertEqual(0, original_state_dict["bar"]["iter_count"])
+        self.assertNotIn("baz", original_state_dict)
+
+        for _ in multi_dataloader:
+            pass
+
+        self.assertEqual(multi_dataloader.individual_dataloaders["foo"].iter_count, 1)
+        self.assertEqual(multi_dataloader.individual_dataloaders["bar"].iter_count, 1)
+
+        new_state_dict = multi_dataloader.state_dict()
+
+        # Load state dict to reset to initial state
+        multi_dataloader.load_state_dict(original_state_dict)
+        self.assertEqual(multi_dataloader.individual_dataloaders["foo"].iter_count, 0)
+        self.assertEqual(multi_dataloader.individual_dataloaders["bar"].iter_count, 0)
+
+        # instantiate a new multi-dataloader with a new different name
+        new_multi_dataloader = MultiDataLoader(
+            {
+                "foo": DummyIterable([1, 2, 3]),
+                "qux": DummyIterable([4, 5, 6]),
+                "baz": [7, 8, 9],
+            },
+            InOrder(),
+        )
+        new_multi_dataloader.load_state_dict(new_state_dict)
+        # foo's count should be loaded correctly
+        self.assertEqual(
+            new_multi_dataloader.individual_dataloaders["foo"].iter_count, 1
+        )
+        # qux's iter_count should still be 0 because it was not in the original state dict
+        self.assertEqual(
+            new_multi_dataloader.individual_dataloaders["qux"].iter_count, 0
+        )

torchtnt/utils/data/multi_dataloader.py

@@ -15,6 +15,7 @@
     DataIterationStrategyRegistry,
     MultiIterator,
 )
+from torchtnt.utils.stateful import Stateful
 
 if TYPE_CHECKING:
     from torch.utils.data import DataLoader
@@ -64,7 +65,7 @@ def __init__(
                     )
 
     def __iter__(self) -> Iterator[Dict[str, Any]]:
-        """Iterator functions for the collection of dataloaders
+        """Iterator functions for the collection of dataloaders.
 
         Returns:
             a newly created iterator based on DataIterationStrategy
@@ -80,3 +81,36 @@ def __iter__(self) -> Iterator[Dict[str, Any]]:
             iteration_strategy=self.iteration_strategy,
         )
         return self.iterator
+
+    def state_dict(self) -> Dict[str, Any]:
+        """Return an aggregated state dict based on individual dataloaders.
+
+        The state dict is keyed off the names provided by ``individual_dataloaders``.
+
+        Note:
+            Only states from dataloaders that implement the :class:`~torchtnt.utils.stateful.Stateful` protocol are included in the returned state dict.
+        """
+        state_dict = {}
+        for name, dl in self.individual_dataloaders.items():
+            if isinstance(dl, Stateful):
+                state_dict[name] = dl.state_dict()
+
+        return state_dict
+
+    def load_state_dict(self, state_dict: Dict[str, Any]) -> None:
+        """Loads aggregated state dict based on individual dataloaders.
+
+        The provided state dict should be keyed off the names provided by ``individual_dataloaders``.
+
+        Note:
+            Only states from dataloaders that implement the :class:`~torchtnt.utils.stateful.Stateful` protocol are loaded.
+        """
+        for name, dl in self.individual_dataloaders.items():
+            if isinstance(dl, Stateful):
+                contents = state_dict.get(name, None)
+                if contents is None:
+                    logger.warning(
+                        f"Skipping loading state dict for dataloader {name} as there is no corresponding entry in the state dict"
+                    )
+                    continue
+                dl.load_state_dict(contents)