Add DropEmAndF1 metric to __init__.py (#3191)

allennlp/training/metrics/__init__.py

@@ -11,6 +11,7 @@
 from allennlp.training.metrics.categorical_accuracy import CategoricalAccuracy
 from allennlp.training.metrics.conll_coref_scores import ConllCorefScores
 from allennlp.training.metrics.covariance import Covariance
+from allennlp.training.metrics.drop_em_and_f1 import DropEmAndF1
 from allennlp.training.metrics.entropy import Entropy
 from allennlp.training.metrics.evalb_bracketing_scorer import EvalbBracketingScorer, DEFAULT_EVALB_DIR
 from allennlp.training.metrics.fbeta_measure import FBetaMeasure