Add STATUSES_EXPECTING_DATA to trials consts (facebook#2621)

Summary:
Pull Request resolved: facebook#2621

We'll want to leverage this in a few places in various generation strategies we are creating.

This provides an easy to use constant for which trial statuses are those that we expect to receive data from.

Reviewed By: saitcakmak

Differential Revision: D60485193

fbshipit-source-id: 81a77047432469bf67c09f1fe867c702291d1c7f

Author: mgarrard

ax/core/base_trial.py

@@ -102,11 +102,7 @@ def is_terminal(self) -> bool:
     @property
     def expecting_data(self) -> bool:
         """True if trial is expecting data."""
-        return (
-            self == TrialStatus.RUNNING
-            or self == TrialStatus.COMPLETED
-            or self == TrialStatus.EARLY_STOPPED
-        )
+        return self in STATUSES_EXPECTING_DATA
 
     @property
     def is_deployed(self) -> bool:
@@ -168,6 +164,12 @@ def __repr__(self) -> str:
 
 NON_ABANDONED_STATUSES: Set[TrialStatus] = set(TrialStatus) - {TrialStatus.ABANDONED}
 
+STATUSES_EXPECTING_DATA: List[TrialStatus] = [
+    TrialStatus.RUNNING,
+    TrialStatus.COMPLETED,
+    TrialStatus.EARLY_STOPPED,
+]
+
 
 # pyre-fixme[24]: Generic type `Callable` expects 2 type parameters.
 def immutable_once_run(func: Callable) -> Callable: