[Example] Super small fix for import in HGL_SP example (#6984)

Co-authored-by: Hongzhi (Steve), Chen <chenhongzhi.nkcs@gmail.com>

examples/pytorch/hgp_sl/functions.py

@@ -9,10 +9,10 @@
 """
 import dgl
 import torch
+from dgl._sparse_ops import _gsddmm, _gspmm
 from dgl.backend import astype
 from dgl.base import ALL, is_all
 from dgl.heterograph_index import HeteroGraphIndex
-from dgl.sparse import _gsddmm, _gspmm
 from torch import Tensor
 from torch.autograd import Function