Fix error in BooleanAccuracy when total count is 0 (#2991)

* Fix error when count is 0

* Update boolean_accuracy.py

Author: abhishek0318

allennlp/tests/training/metrics/boolean_accuracy_test.py

@@ -64,3 +64,7 @@ def test_incorrect_mask_shape_catches_exceptions(self):
         incorrect_shape_mask = torch.randint(0, 2, [5, 8])
         with pytest.raises(ValueError):
             accuracy(predictions, labels, incorrect_shape_mask)
+
+    def test_does_not_divide_by_zero_with_no_count(self):
+        accuracy = BooleanAccuracy()
+        self.assertAlmostEqual(accuracy.get_metric(), 0.0)

allennlp/training/metrics/boolean_accuracy.py

@@ -84,7 +84,10 @@ def get_metric(self, reset: bool = False):
         -------
         The accumulated accuracy.
         """
-        accuracy = float(self._correct_count) / float(self._total_count)
+        if self._total_count > 0:
+            accuracy = float(self._correct_count) / float(self._total_count)
+        else:
+            accuracy = 0.0
         if reset:
             self.reset()
         return accuracy