[tune] Trial reporter fix (#3951)

Fixes #3949.

python/ray/tune/function_runner.py

@@ -8,7 +8,7 @@ import threading
 
 from ray.tune import TuneError
 from ray.tune.trainable import Trainable
-from ray.tune.result import TIMESTEPS_TOTAL
+from ray.tune.result import TIMESTEPS_TOTAL, TRAINING_ITERATION
 
 logger = logging.getLogger(__name__)
 
@@ -28,6 +28,7 @@ class StatusReporter(object):
         self._lock = threading.Lock()
         self._error = None
         self._done = False
+        self._iteration = 0
 
     def __call__(self, **kwargs):
         """Report updated training status.
@@ -44,6 +45,7 @@ class StatusReporter(object):
 
         with self._lock:
             self._latest_result = self._last_result = kwargs.copy()
+        self._iteration += 1
 
     def _get_and_clear_status(self):
         if self._error:
@@ -55,10 +57,13 @@ class StatusReporter(object):
                            "last result. To avoid this, include done=True "
                            "upon the last reporter call.")
             self._last_result.update(done=True)
+            self._last_result.setdefault(TRAINING_ITERATION, self._iteration)
             return self._last_result
         with self._lock:
             res = self._latest_result
             self._latest_result = None
+            if res:
+                res.setdefault(TRAINING_ITERATION, self._iteration)
             return res
 
     def _stop(self):