@shucheng: create the example codes for end-to-end pipeline training/testing for tree decoder

[teddylfwu]

• create the example codes for end-to-end pipeline training/testing for tree decoder
• requirements:
  • find a NLP task requiring tree decoder such as semantic parsing/word math problems or other popular tasks
  • fully test all graph construction methods (static and dynamic)
  • fully test all graph embeddings methods (ggnn, graphsage, gat)

[teddylfwu]

• fully execute all requirements