fix: Database now contains an expected all_samples

Corresponds to the bob.bio.base equivalent, allowing use of pad datasets
with bio commands (like bob bio annotate).

src/bob/pad/base/pipelines/abstract_classes.py

+from __future__ import annotations
+
 from abc import ABCMeta, abstractmethod
 
+from bob.pipelines import Sample
+
 
 class Database(metaclass=ABCMeta):
     """Base database class for PAD experiments."""
 
     @abstractmethod
-    def fit_samples(self):
+    def fit_samples(self) -> list[Sample]:
         """Returns :any:`bob.pipelines.Sample`'s to train a PAD model.
 
         Returns
@@ -16,7 +20,7 @@ class Database(metaclass=ABCMeta):
         pass
 
     @abstractmethod
-    def predict_samples(self, group="dev"):
+    def predict_samples(self, group: str = "dev") -> list[Sample]:
         """Returns :any:`bob.pipelines.Sample`'s to be scored.
 
         Parameters
@@ -30,3 +34,20 @@ class Database(metaclass=ABCMeta):
             List of samples to be scored.
         """
         pass
+
+    def all_samples(
+        self, groups: str | list[str] | None = None
+    ) -> list[Sample]:
+        """Returns all the samples of the database in one list.
+
+        Giving ``groups`` will restrict the ``predict_samples`` to those groups.
+        """
+        samples = self.fit_samples()
+        if groups is not None:
+            if type(groups) is str:
+                groups = [groups]
+            for group in groups:
+                samples.extend(self.predict_samples(group=group))
+        else:
+            samples.extend(group=None)
+        return samples