only filling zeros for min/max reducer (#2026)

python/dgl/ops/spmm.py

@@ -62,6 +62,8 @@ def gspmm(g, op, reduce_op, lhs_data, rhs_data):
     ret = gspmm_internal(g._graph, op,
                          'sum' if reduce_op == 'mean' else reduce_op,
                          lhs_data, rhs_data)
+    # Replace infinity with zero for isolated nodes when reducer is min/max
+    if reduce_op in ['min', 'max']:
         ret = F.replace_inf_with_zero(ret)
 
     # divide in degrees for mean reducer.