add GIN model

[rong0525]

We construct Graph Isomorphism Network (GIN), and the relevant paper comes from 'Keyulu Xu, Weihua Hu, Jure Leskovec, et al . How Powerful are Graph Neural Networks?[C]. //In International Conference on Learning Representations(2019).'

[Zhanghyi]

please write a document in the "openhgnn/output" directory that provides a brief introduction to how the GIN model has been modified to a heterogenous version, and also includes some examples of command line usage.

and please include formulas for both homo and hetero GIN