add new features:
* NAS
* sampling
* Link prediction

Update version (functional)

Revise example and documents for sampling

Revise model tutorial and add some default hyper parameters.

Fix bugs in nas, revise example.

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.

Add nas support to node classification. Including:
* space: singlepath, graphnas, graphnasmacro
* algo: random, enas, darts, rl
* estimator: oneshot, scratch

* Add sampling support for AutoGL, including nodewise, layerwise and subgraph sampling.
* Refactor part of the model and trainer.

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

add link prediction into documents