[DeprecationWarning] Fixing the warnings appearing in test_traversal.py::test_bfs.

[drivanov]

Description

The following two sets of warnings that appear in test_traversal.py::test_bfs have been fixed:

./../../usr/local/lib/python3.10/dist-packages/dgl/graphbolt/base.py:80
  /usr/local/lib/python3.10/dist-packages/dgl/graphbolt/base.py:80: DeprecationWarning: torch.library.impl_abstract was renamed to torch.library.register_fake. Please use that instead; we will remove torch.library.impl_abstract in a future version of PyTorch.
    @torch.library.impl_abstract("graphbolt::expand_indptr")

tests/python/common/test_traversal.py::test_bfs[idtype0]
tests/python/common/test_traversal.py::test_bfs[idtype1]
  /usr/local/lib/python3.10/dist-packages/networkx/utils/backends.py:812: DeprecationWarning:
 
  random_tree is deprecated and will be removed in NX v3.4
  Use random_labeled_tree instead.
    return self.orig_func(*args, **kwargs)

https://networkx.org/documentation/stable/developer/deprecations.html#version-3-4

Checklist

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [$CATEGORY] (such as [NN], [Model], [Doc], [Feature]])
• 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

Changes

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