[NN] nn modules & examples update

[yzh119]

Description

This PR implements some nn modules in MXNet as we did in PyTorch.

Other updates:

• fixed the memory consumption problem of GGNN mentioned in Memory consumption of the GGNN module #852 .
• add MXNet examples of appnp, graphsage, sgc, monet, gin.
• add PyTorch examples of chebnet & monet on MNIST.
• minor changes with APIs.

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