Remove overly restrictive check to support early stopping with BLEU (#…

…384)

* Remove overly restrictive check to support early stopping with BLEU

Removes an overly restrictive check introduced with the train refactoring.
Re-allows early stopping w.r.t BLEU.

* changelog

CHANGELOG.md

@@ -10,6 +10,10 @@ Note that Sockeye has checks in place to not translate with an old model that wa
 
 Each version section may have have subsections for: _Added_, _Changed_, _Removed_, _Deprecated_, and _Fixed_.
 
+## [1.18.10]
+### Fixed
+- Re-allow early stopping w.r.t BLEU
+
 ## [1.18.9]
 ### Fixed
 - Fixed a problem with lhuc boolean flags passed as None.

sockeye/__init__.py

@@ -11,4 +11,4 @@
 # express or implied. See the License for the specific language governing
 # permissions and limitations under the License.
 
-__version__ = '1.18.9'
+__version__ = '1.18.10'

sockeye/training.py

@@ -880,7 +880,6 @@ def _check_args(self,
         """
         Helper function that checks various configuration compatibilities.
         """
-        utils.check_condition(early_stopping_metric in metrics, "Early stopping metric must be tracked.")
         utils.check_condition(len(metrics) > 0, "At least one metric must be provided.")
         for metric in metrics:
             utils.check_condition(metric in C.METRICS, "Unknown metric to track during training: %s" % metric)