Fix bug leading to mean and var being NaN if calculated distributed

heat/core/statistics.py

@@ -1120,6 +1120,8 @@ def __merge_moments(
     if len(m1) != len(m2):
         raise ValueError(f"m1 and m2 must be same length, currently {len(m1)} and {len(m2)}")
     n1, n2 = m1[-1], m2[-1]
+    if (n2 == 0).all():
+        return m1
     mu1, mu2 = m1[-2], m2[-2]
     n = n1 + n2
     delta = mu2 - mu1

tests/core/test_statistics.py

@@ -1599,3 +1599,11 @@ def test_var(self):
 
         # edge case from #2374
         self.assertEqual(ht.var(ht.array([0.], split=None), axis=0, ddof=0), 0)
+
+    # meant to be run on four parallel processes (mpirun -n 4)
+    def test_first_two_leading_ranks_empty(self):
+        data_all = ht.arange(100., split=0)
+        data = data_all[data_all >= 50.]
+
+        self.assertEqual(ht.mean(data), 74.5)
+        self.assertEqual(ht.var(data), 208.25)