Adding tutorials for negative sampling and dataset evaluation

[shenyangHuang]

add the following tutorials to docs/tutorials/:

• how to use the negative sampler, loading the splits, querying it etc.
• [can be same tutorial as above] How to use the evaluator
• how to query node labels for node prop pred task

[shenyangHuang]

see docs/tutorials/Edge_data_numpy.ipynb as an example for how to construct jupyter notebook tutorials.
Note that this also requires optional dependencies for jupyter notebook as specified in the README documentation