[Feature] Edge Group Apply API

[VoVAllen]

Description

Add group apply API
Refer to #332
group_apply_edges(self, group_by, func)

Checklist

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [$CATEGORY] (such as [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

• Add edge group apply API
• degree bucketing schedule for group apply
• lowering edge grouping by node to c++
• Add softmax to test backend
• test cases

[lingfanyu]

@VoVAllen I finished c++ part and rearranged group_apply scheduler code.

[VoVAllen]

Thanks! Looks good to me!

[VoVAllen]

Looks ready to merge!

[lingfanyu]

@VoVAllen Thanks!