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

change model function name decode to lp_decode in response to PR #31

* add link prediction trainer, solver, and model adaptation

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