[Example] Implement LINE with dgl and pytorch

[ShawXh]

Description

Implement a network embedding algorithm, LINE (https://arxiv.org/pdf/1503.03578). The implementation has been tested on several ogbn datasets. This PR also solves the problems referred in last unsuccessful PR #1968.

The experimental results are as following:

ogbl-arxiv
#params: 33023343(model) + 142888(mlp) = 33166231
Highest Train: 82.94 ± 0.11
Highest Valid: 71.76 ± 0.08
Final Train: 80.74 ± 1.30
Final Test: 70.47 ± 0.19

obgl-proteins
#params: 25853524(model) + 129648(mlp) = 25983172
Highest Train: 93.11 ± 0.04
Highest Valid: 70.50 ± 1.29
Final Train: 77.66 ± 10.27
Final Test: 62.07 ± 1.25

ogbl-products
#params: 477570049(model) + 136495(mlp) = 477706544
Highest Train: 98.01 ± 0.32
Highest Valid: 89.57 ± 0.09
Final Train: 94.96 ± 0.43
Final Test: 72.52 ± 0.29

The results are also given in README.md.

Checklist

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [$CATEGORY] (such as [NN], [Model], [Doc], [Feature]])
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented
• To the my best knowledge, examples are either not affected by this change,
  or have been fixed to be compatible with this change
• Related issue is referred in this PR

Changes