Add active arms property (facebook#3286)

Summary:

I've been doing this by hand often lately

Reviewed By: mgarrard

Differential Revision: D68844139

Author: Daniel Cohen

ax/core/base_trial.py

@@ -585,6 +585,11 @@ def abandoned_arms(self) -> list[Arm]:
         """All abandoned arms, associated with this trial."""
         pass
 
+    @property
+    def active_arms(self) -> list[Arm]:
+        """All non abandoned arms associated with this trial."""
+        return [arm for arm in self.arms if arm not in self.abandoned_arms]
+
     @abstractproperty
     def generator_runs(self) -> list[GeneratorRun]:
         """All generator runs associated with this trial."""

ax/core/tests/test_batch_trial.py

@@ -413,6 +413,8 @@ def test_AbandonArm(self) -> None:
         metadata = self.batch.abandoned_arms_metadata[0]
         self.assertEqual(metadata.reason, reason)
         self.assertEqual(metadata.name, arm.name)
+        self.assertEqual(len(self.batch.active_arms), len(self.arms) - 1)
+        self.assertNotIn(arm, self.batch.active_arms)
 
         # Fail to abandon arm not in BatchTrial
         with self.assertRaises(ValueError):