[Model]Mg2vec - Githubissues

Description

Add the Mg2vec Model and add the EdgeClassification Task

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 best of my 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
[ ] If the PR is for a new model/paper, I've updated the example index here.

[ ] Add configs for Mg2vec in config.ini and config.py
[ ] Add Mg2vec.py, which contains the model part
[ ] Add mg2vec_sampler.py for reading data
[ ] Add mg2vec_trainer.py for training
[ ] Add EdgeClassificationDataset.py for EdgeClassification Task, which is a modified version of NodeClassificationDataset.py
[ ] Add mg2vec_dataset.py for download/read mg2vec dataset
[ ] Add edge_classification.py, which is a modified version of node_classification.py
[ ] Add ec_with_SVC function in evaluator.py for edge_classification task
[ ] Add readme.md for Mg2vec model
[ ] Modify corresponding init.py and experiment.py