Add barebones Artifact abstraction

Adds the base interface without supplying a default implementation.
After it is checked in, we can think about whether we want to add a base
implementation, e.g. via fsspec.

src/nnbench/types.py

@@ -1,8 +1,11 @@
 """Useful type interfaces to override/subclass in benchmarking workflows."""
 from __future__ import annotations
 
+import os
 from dataclasses import dataclass, field
-from typing import Any, Callable, TypedDict
+from typing import Any, Callable, Generic, TypedDict, TypeVar
+
+T = TypeVar("T")
 
 
 class BenchmarkResult(TypedDict):
@@ -14,6 +17,42 @@ def NoOp(**kwargs: Any) -> None:
     pass
 
 
+class Artifact(Generic[T]):
+    """
+    A base artifact class for loading (materializing) artifacts from disk or from remote storage.
+
+    This is a helper to convey which kind of type gets loaded for a benchmark in a type-safe way.
+    It is most useful when running models on already saved data or models, e.g. when
+    comparing a newly trained model against a baseline in storage.
+
+    Subclasses need to implement the `Artifact.materialize()` API, telling nnbench how to
+    load the desired artifact from a path.
+
+    Parameters
+    ----------
+    path: str | os.PathLike[str]
+        Path to the artifact files.
+    """
+
+    def __init__(self, path: str | os.PathLike[str]) -> None:
+        # Save the path for later just-in-time materialization.
+        self.path = path
+        self._value: T | None = None
+
+    @classmethod
+    def materialize(cls) -> "Artifact":
+        """Load the artifact from storage."""
+        raise NotImplementedError
+
+    def value(self) -> T:
+        if self._value is None:
+            raise ValueError(
+                f"artifact has not been instantiated yet, "
+                f"perhaps you forgot to call {self.__class__.__name__}.materialize()?"
+            )
+        return self._value
+
+
 # TODO: Should this be frozen (since the setUp and tearDown hooks are empty returns)?
 @dataclass(init=False)
 class Params: