Segmentation fault if run `tests/mxnet/test_nn.py` on cpu with python 3.7.0

[Rhett-Ying]

🐛 Bug

it works well with python 3.6.9. seems related to dgl.add_self_loop() in below code: https://github.com/dmlc/dgl/blob/39987bc59970289b774319f9e7ccd1ad57a1cb98/tests/test_utils/graph_cases.py#L92-L97

Related PR: https://github.com/dmlc/dgl/pull/4406

To Reproduce

Steps to reproduce the behavior: docker run -it dgllib/dgl-ci-cpu:v220816 /bin/bash conda activate mxnet-ci clone/build/install DGL DGLBACKEND=mxnet PYTHONPATH=tests:$PYTHONPATH python -m pytest tests/mxnet/test_nn.py::test_graph_conv2

Expected behavior

Environment

• DGL Version (e.g., 1.0): master, 0.9
• Backend Library & Version (e.g., PyTorch 0.4.1, MXNet/Gluon 1.3): mxnet 1.6.0
• OS (e.g., Linux): Linux
• How you installed DGL (conda, pip, source):
• Build command you used (if compiling from source):
• Python version: 3.7.0
• CUDA/cuDNN version (if applicable):
• GPU models and configuration (e.g. V100):
• Any other relevant information:

Additional context

[github-actions[bot]]

This issue has been automatically marked as stale due to lack of activity. It will be closed if no further activity occurs. Thank you