[Feature] Self-supervised Learning on Graphs

[icycookies]

Description

• modify evaluation metric for agc
• new ssl models: m3s, gae/vgae, supergat
• modifications: mvgrl-sparse, s2grl(sampling), self-auxiliary-task(different training schemes)
• dgi: fail to reproduce reddit models with graphsage

Checklist

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [$CATEGORY] (such as [Model], [Doc], [Feature], [Bugfix])
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented
• To 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