Modify GIN encoder&decoder for DGL Backend. Pass test performance check

Debug for Link Prediction for PyG Backend

The words *mendatory* and *mandatory* have different meanings, take a look at dictionary for more details. I replaced the *mendatory* (possibly a typo for word *mandatory*) with *required*. The pair `optional <-> required` is more commonly used in programming.

Merge from refactor_test branch and already fixed bugs detected through tests for refactor, and temporarily override  method of dataset provided by PyTorch-Geometric to maintain compabillity with PyTorch-Geometric>=1.7.0