[BUG] fix for zero degree nodes (#2786)

* fix for zero degree nodes

* Update python/dgl/ops/spmm.py

linting fix
Co-authored-by: Zihao Ye <zihaoye.cs@gmail.com>
Co-authored-by: Konstantin Sofeikov <konstantin.sofeikov@complyadvantage.com>
Co-authored-by: Zihao Ye <expye@outlook.com>
Co-authored-by: Zihao Ye <zihaoye.cs@gmail.com>

python/dgl/ops/spmm.py

@@ -70,7 +70,7 @@ def gspmm(g, op, reduce_op, lhs_data, rhs_data):
     if reduce_op == 'mean':
         ret_shape = F.shape(ret)
         deg = g.in_degrees()
-        deg = F.astype(F.clamp(deg, 1, g.number_of_edges()), F.dtype(ret))
+        deg = F.astype(F.clamp(deg, 1, max(g.number_of_edges(), 1)), F.dtype(ret))
         deg_shape = (ret_shape[0],) + (1,) * (len(ret_shape) - 1)
         return ret / F.reshape(deg, deg_shape)
     else: