Fixing problem with complex numbers appearing in the lap_pe function.

[drivanov]

Description

As I could see from test_nn.py::test_dgn_conv[16-2.5-scalers0-aggregators0-16-16], the complex numbers can appear in lap_pe in both cases, when k + 1 < n - 1 is TRUE or FALSE. Therefore, converting them to a real number should be done for both cases.

By doing this, we will resolve the following warning and potential crash in NN code

tests/python/pytorch/nn/test_nn.py::test_dgn_conv[16-2.5-scalers0-aggregators0-16-16]
  /usr/local/lib/python3.10/dist-packages/dgl/backend/pytorch/tensor.py:53: UserWarning: Casting complex values to real discards the imaginary part (Triggered internally at /opt/pytorch/pytorch/aten/src/ATen/native/Copy.cpp:299.)
    return th.as_tensor(data, dtype=dtype)

Checklist

Please feel free to remove inapplicable items for your PR.

• I've leverage the tools to beautify the python and c++ code.
• The PR is complete and small, read the Google eng practice (CL equals to PR) to understand more about small PR. In DGL, we consider PRs with less than 200 lines of core code change are small (example, test and documentation could be exempted).
• To the best of my knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

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