GCMC Implementation

[darrylong]

Description

Implemented GCMC (Graph Convolutional Matrix Completion) using PyTorch and DGL library.

Checklist:

• I have added tests.
• I have updated the documentation accordingly.
• I have updated README.md (if you are adding a new model).
• I have updated examples/README.md (if you are adding a new example).
• I have updated datasets/README.md (if you are adding a new dataset).

[tqtg]

@darrylong let's move all the import statements of additional dependencies (e.g., dgl, torch) inside the functions that require them. We don't have some dependencies pre-installed.

[darrylong]

@tqtg Reorganized codes for better readability, with inner import statements on recom_gcmc.py file. Readmes updated as well.